I do not write fan fiction, but I love to read it, particularly canon-based, post-DH Harry Potter stories. I do, however, enjoy AU stories, as long as the characterizations are still believable. Every now and then I'll give a non-canon story a go, particularly if it's written by an author I respect.
I am a picky reader, and I'm only interested in reading fics by authors who actually put some thought and care into their work. If I enjoy your work, I will definitely let you know. I believe in writing thorough reviews, so even if I adore your writing, I'll still write more than a generic "Good job." However, I don't like feeling that an author is holding their story hostage until they receive an arbitrary number of reviews. If I see an Author's Note that implies this is the way a story is being handled, I'm probably not going to write a review.
Kudos to authors who:
1. Reply to every review: I didn't even realize some authors do this until recently, but I have to say it's quite the lovely practice. I would say that some very popular authors may not have the time to do this, but to be honest, some of the writers who've personally thanked are popular authors who've received 100+ reviews for their work. I obviously realize published authors don't do this, but this is a fan fiction community!
2. Update consistently: I know this is a forum for hobbyists with day jobs and other ongoing responsibilities, but if you're writing a multi-chapter fic, please update at least once every two-to-four weeks. If you can't, or if you've decided to abandon your fic, please give readers some vague idea of how long it's going to be before we can expect another chapter.
3. Have their stories Beta'd: Not every good writer is a good speller, but please, please please spell-check your work. Not only that, please read it over (or better yet, have someone else copy-edit it) to look for mistakes that auto spell-checks can't find. It really takes a reader "out of the story" if every other sentence includes a missed word, a grammatical error and the like. The only exception to this is writers for whom English is a second language, in which case, I'm generally too impressed they even wrote in English to care much.
4. Fade to black: Some authors are adept at writing sex scenes, but if you're having trouble in that area, then describe the characters' feelings or what they say to each other instead of trying to write some cringe-inducing explicit sex scene that is basically a regurgitation of something you read in another fic. Again, some authors are masters of smut, but for many authors less can be more when it comes to good sex (on paper, anyway).