I'm tired of hearing that fanfic is easier to write than original fiction. It's not a training ground for beginners. It's not a kit with all the parts pre-cut. Well-done fanfic is harder to write than original fiction in a handful of ways, which I am going to rant about.
1) The boundaries of what you can decide to do are less flexible. Any story operates by the rules of its fictional world, and yes those rules can be plot-starters as well as constraints, but sometimes they are limiting and confining. The creator, not you, has chosen those rules, and like it or not you're stuck with them (unless you write AU, but then I suppose you'll have the stigma of writing AU to contend with, which means you have one more notch of bias against you in some circles, and you have to be that one notch better to get people to like your fic.) Readers will expect you to stick reasonably close to canon, and if you don't, they'll expect you to have a good reason why it makes the story better if you don't. So not only do you have to know and understand canon extremely well-- even though you didn't make it up, and you might not be able to guess the creator's intentions-- you also have to work mostly with this given set of principles, some of which you may want to work with and some of which you may not. You have to craft a good story and you have to do it in a way so that other fans agree that they can indeed picture the world as being the way you claim it is.
A common objection to this is, "But original fiction has rules and constraints as well!" Yes, but you make them up yourself. You can choose your own rules, and if you don't feel comfortable working with a certain concept, or you're unfamiliar with it, you don't have to use it. In fanfic, you're pretty much stuck. You'll have to learn about the things you don't know, and you'll have to put yourself through the work of understanding concepts you would otherwise not have bothered with. It can be hard work-- fun and often voluntary, but still extra work, and sometimes hard to get right.
2) Sometimes you run out of places in the plot to fit exploration without violating canon. Gosh, three out of my four favorite characters are dead and the other one is obsessed with one of the dead people. The canon stories are pretty tight-- where am I going to fit in an exploration of the issues that are raised for these characters during the story, after they begin to grapple with the problem but before they die? Well, I can write offcamera scenes, but they'll mostly be one-shot single-scene pieces of about 1500 words. They have to be. Any more, and I'll start running into plot threads that I can't displace. I can't write epics because there isn't enough room.
You can write backstory, but there can be something dissatisfying about writing backstory when you know all you're doing is referring to things the readers have already read about in the future. The suspense just doesn't reach the same level-- your readers already know what's going to happen. They know not only that your characters will get through this trying time, but how they will get through, what it will do to them, and how they will feel about it seven years later. It's like reading a book knowing all the spoilers already.
All of which is not to put down backstory fics or offcamera during-the-story fics. They're just as worthwhile as any other kind of writing. But I do think the author has to work just that much harder to retain the reader's interest when the reader already knows what's going to happen. If you really want to stick to the source material that's made readers want to read in your fic in the first place, you may want to do backstory that stays fairly close to the characters and issues in canon-- and you lose the element of suspense, so your fic has to be that much better; and you have to find ways to make your reader hang on your words and believe that you really do have something valuable to say, even though they THINK they already know how it's going to go.
It can be difficult to prove in a fanfic that you are heading somewhere important, that you have a point, that your exploration of the situation is valuable, when the readers already have an idea of what they think happened.
3) Fans' preconceptions are strict and need to be considered. Whether or not you're trying to offer an alternate explanation that goes against the usual fanon, you've got to deal with the fact that your readers all have varying expectations of the characters you're writing. You have to take all those different preconceptions into account.
If you're going to explore a character in a complex and meaningful way, there are usually multiple ways to interpret that character. And some of your readers are not going to interpret the same way as you. They're going to have a nagging feeling in their heads that when they saw the canon, they thought something else. Or that they and twenty-seven other fans they've talked to all agree that canon should be interpreted in this other way. They may have already had long discussions picking apart what they saw in the character and why their interpretation works "best." You, the writer, are most likely struggling against that for at least some of your readers. There's no one interpretation that's going to be universal, that's not going to set the alarm off in someone's head. You have to be taking all those potential alarms into account. And to do this, you have to establish why your character is that way, so well that even a reader with strong preconceptions (and perhaps even a bias directly against your idea) will nonetheless think, "All right, I can buy this as another possible explanation, no matter how sure I am of my own."
This goes for plot points, too. If most of the fandom believes that things happened a certain way, but you see them as having been slightly different, it's going to be an uphill battle to convince anyone that your fic makes sense-- because not only does everyone have fanviews, but when they all get together and fangeek and discover that most others have the same fanview on a given topic, it combines to form a Giant Ultra Super-Fanview of Doom that fans take for a given. This is fanon that hasn't really been created in a way that anyone notices or pays attention to, so people may not even be aware of it, but it gets subtly in everyone's consciousness anyhow because most people saw it that way for some reason, and you didn't. You'll need to be fantastically skilled at showing things another way, even if your own view would be, without the existence of the fanon, quite plausible.
Intelligent, open-minded readers will still accept your alternate views, provided you defend them. But even the most accepting reader who encounters a differing opinion is going to have a slight bias in another direction. That reader has thought about a character for a long time, and already has a way of thinking that makes sense. In order to get that person to see your view, you have to pull them not from a neutral standpoint to yours, but from a differing standpoint to yours, which is a slightly longer distance you've got to pull them. Slightly harder work. And we're talking about a widely accepting reader, here, which many readers in many fandoms are not. The best fics have a broad appeal, and can be enjoyed even by those who enter the act of fanfic reading with reservations and biases.
4) Fans are less accepting of changes in characters. I don't mean AU; I mean character development or differing interpretations.
In original source material, characters sometimes change-- a lot. Hopefully in ways that make sense. But every little detail of their change doesn't need to be documented; it just needs to make sense. The readers will be trusting and forgiving; they know that you've been writing this character all along, and that you know how the character was at the start (because, of course, you wrote the character,) and that the changes you make are most likely intentional. You can allow your characters to grow and evolve dramatically, and people are less likely to criticize the way you change them because they see it as character development rather than sudden OOCness. Not so much in fanfic. You're going to face a group of people who know the original character very, very well and who want to see you stick to that. And many fans are often so used to seeing crappy OOC fanfic that they're a bit trigger-happy with that accusation. You've got to do an extremely good job of proving your work IC-- sometimes you're assumed guilty of OOCness at the slightest step sideways.
It's rather understandable-- the fans reading the fic loved the source, and the more you deviate from the source, the less you're writing about what they loved. But it's another thing you have to work against. You have to make sure your changes are so well detailed that, although the fans may instinctively react with "Whoa! I liked the character better in canon," they also can admit "but I think the change made sense."
Basically, you're writing for readers who are biased, and naturally inclined to be less forgiving of changes-- you've got to work very hard to prove yourself. And when you do it, it's beautiful. It's hard work that one can enjoy doing, but it's still hard work and finicky to pull off.
5) You have a lot to live up to. Those who are reading your fanfic are doing so, most likely, out of interest in and respect for the source material. They want more, and they want it to live up to this source that they already really love. Your fic is measuring up against the thing the fan is devoted to. That's a tall order.
Readers don't want to be disappointed. They're seeking, in their hearts, the spirit of the source material. They may not expect your fic to be as good as canon-- or, depending on the fandom, they may know that canon is already seriously lacking in quality. However, they expect quality from your fic no matter what, because they certainly respect or care about the fandom in some way (why else would they be reading?) and they want your fanfic to do justice, not necessarily to the original source material, but to their own love for it. Nothing will anger a fan so much as seeing something they love being treated disrespectfully. Even if you don't go so far as trashing or bashing, a boring treatment of something can disappoint your readers for similar reasons-- it may not provoke the righteous anger that bashing causes, but it can leave fans feeling empty. A truly good fanfic does justice to all the awesome things in the original source.
6) Not getting to write the big important scenes. Sometimes your fic is actually about a scene that takes place in canon. You're trying to show how someone became a certain way, or how a conflict built up, or what caused a later explosion. Or you're showing the effects of something that happened, what was going on in someone's head, the mess that had to be cleared up, the confusing things people came away believing. Your fic may not contain the scene that takes place in canon (rewriting such a scene can be awkward,) and yet that scene is the thing that inspired your writing.
So then you have to make your piece extra-good to make up for the fact that you didn't get to write the cool scene. You have to have rising action, climax and denouement, just the same as in any other piece of fiction-- and it can't be overshadowed by the greatness of the scene you're referring to; it has to have its own power and emotion.
Again, a challenge. Again, a fun challenge! But again, a difficulty you don't really run into with original fiction, in which the author always gets to write all the most important scenes in the story.
These things make writing harder. A great fanfic-- not just a decent one, but a truly amazing work-- has to reach a standard on all these things that is, in some ways, less forgiving than the general standard for original fiction. Many of your readers are reading fanfic because they know the original source very well already and they're looking for more-- but they know the source so well that they're capable of spotting and picking at any flaw. That's natural; they love the source, right? So if they see something that violates the source, THEY ARE GOING TO NOTICE. And it's going to bug them, more than it would bug them if they didn't love the source, because the writer is spreading badwriterness on a thing they love. They're going to expect that a good fanfic be really awesome, because it's a homage to something they love, and to be a good homage it had better be worthy.
One thing that I frequently hear said to support the claim that original fiction is harder to write than fanfic is that in original fic you've got to worldbuild everything, whereas in fanfic you only have to fill in gaps. Well, yes, you do-- and worldbuilding is hard work. I don't mean to downplay that; I write original fantasy novels as well as fanfic, and I know just how much work it can be. But I'm not convinced that this one thing overcomes all the other objections on this list automatically. For one thing, it varies depending on the writer's strengths-- some writers don't find worldbuilding hard at all.
For another, worldbuilding may take a lot of thought and research, but the fact that you have to do may not be a strike against you as you struggle to improve the quality of your writing from good to great. Many of the objections on this list are actual impediments to creating a good story, not just because they are extra work, but because you have to curve your story around with them and grapple with them. It's not just a matter of doing the extra work of dancing farther across the stage; it's a matter of dancing across the stage while trying to avoid the giant cardboard trees. Some people may be natural tree-avoiders, but find they're exhausted after making it across the stage, and think that is harder. Others may be willing to undertake the trip across the stage, but find it difficult to avoid the trees gracefully. Whether you think it's harder to do the extra work or to deal with the obstacles is a matter of your individual strengths as a writer, and you cannot make a blanket claim that one is easier than the others for everyone.
Fanfic may provide a "starting ground" for writing-- which is what people always say when they claim that fanfic is easier to write-- but you can just as easily rip ideas from various sources to provide a "starting ground" for original fic (stealing definitively is plagiarism; stealing loosely is inspiration.) I'm not making the claim that fanfic is necessarily harder to write in every way. But it is harder in many ways, and it sure isn't a free ride. People need to stop dismissing fanfic as being a lesser achievement simply by reason of being fanwork. It's not a smaller matter by definition. For some writers, it is the greater achievement and the greater glory.