I would love to hear more about your feelings on typical fandom AUs - you always have interesting perspectives. I didn't even think about the coffeeshop AU power-imbalance problems until now. Only if you'd like to talk more about fandom AUs, though.
First and foremost: I love fan fiction. Hopefully that’s self-evident, given how much fic I read and write, but I’ll say it anyway. I think, as a medium, fic has produced works which are better than most works of original fiction, but that more importantly fic is its own medium, and there’s really no reason to make comparisons to original fiction. I describe fic as a medium not a genre because IMHO fan fiction is its own form of storytelling, just like novels and movies and poems are their own forms of storytelling that can be subdivided into genres.
Fic has unique potential to enrich a source work and to change readers’ views of a source work forever. Some of the best works of fic are the ones that use the uniquenesses of the medium — the audience’s knowledge of the source work, the lack of exposition, the in-fandom tropes — to subvert expectations in ways that are shocking or funny or fascinating.  Case in point, the drabble (a complete story in exactly 100 words) rarely functions alone, but works beautifully as a form of fan fiction, simply because fic done right has no need for exposition or even scene-setting. I could go on.
HOWEVER. Fan fiction, by definition, has no gatekeepers. That’s a strength of the medium to the extent that progressive content prevails and plots which would never make it into mainstream fiction can be openly published. That’s a weakness to the extent that plots which would never make it into mainstream fiction can be openly published. The bravest genre-bending world-twisting fic is more audacious than the bravest published work. The worst-written fic is also more badly written than the worst published work. These are side effects of the lack of gatekeepers, which allows fic to be anarchistic and pluripotent and beautiful and atrocious.
Fan fiction also requires no minimum credentials from its writers. Thus, fic often acts as a sandbox for those of us unable to build castles. That means that some of today’s most successful writers — N.K. Jemison, D.J. MacHale, Scott Westerfeld, Neil Shusterman, Holly Black — have openly discussed getting their start by writing fic. That also means that fic, including fic published on sites such as AO3, is rife with the most common errors of amateur writers.
One of the errors that I find most irksome (although not as much as my peeve about overuse of epithets) is the use of out-of-the-box plots to force characters to act, rather than allowing the characters to make their own decisions and then building the plot from there. Not every single mirrorverse AU or hookerfic or Chosen One AU falls into the problem of having all of the characters’ actions dictated in advance. Not every college AU or fusion or modern AU changes the characters so much that they become unrecognizable puppets with the same names as people from canon. However, a large subset of these AUs do fall into that trap. At that point, the story can have some unique and/or enjoyable qualities… or it can be an out-of-the-box plot with essentially unrecognizable characters, which (if you ask me) isn’t even a work of fan fiction at all.
The specific subset of this error that I find most irksome of all is when an out-of-the-box romantic plot becomes distinctly unromantic when plot or circumstance forces a relationship rather than allowing characters to choose each other. Soul mate AUs, omegaverses, pretend dating, life debts, slavery AUs, cuddling for warmth, and (m)preg AUs can be done without these issues and have been done without these issues. Many of them, however, run into problems with characters being railroaded by plot into relationships that they did not have the chance to choose organically from a set of potentially viable alternatives. At best, this kind of railroading sacrifices character for (cliché) plot. At worst, this kind of railroading has incredibly disturbing consent issues that often go unconsidered by the story itself. Also, yes, too many coffee shop AUs basically depict sexual harassment as meet-cute.
Anywhoo, I love fan fiction. I also use AO3′s filters avoid most of the super-popular fic tropes. Some of my favorite works that make use of the strengths of the genre in unique and mind-bending ways are listed here.
