I only discovered fan fiction two years ago or so and became quickly obsessed. I love to read and have found that many authors, here and among other similar sites, that have helped feed my obsession.
I like Harry potter fiction, primarily because I was very unsatisfied with the last few books and also because I have a love of dark characters and reading works where they are fleshed out is highly enjoyable to me. I have also recently found myself reading a lot of xmen stories, I was never much of a fan but after reading a particularly good Rogue/Magneto fic I was hooked. Other fandoms I like are Star Trek, bothTOS and reboot.
Have a look at my bookmarks if it seems to you that we have complimentary tastes but I do warn you that I do not like fluff, there are few lighthearted fics amongst my favorites and my bookmarks. I should also mention I read both slash and het and most are heavy on the smut, many have a lot of bdsm.
I like dark!fics and AUs but really try to stay away from poorly written characterizations and too much oocedness. I like stories that have a serious tone and a well thought out plot, most are long chaptered fics.
Harry Potter: HP/Voldemort, HP/DM some HP/LM and HP/SS. HG/Voldemort, HG/SS, HG/LM. Really stories with slytherins in general. I like stories where Voldemort is Voldemort, ie: snake like crazy powerful motherf* not TRJ, although I have read a few of the latter that were good. I really enjoy stories where dark magic is expanded upon and the political views and motivations of dark wizards are fleshed out.
Xmen: Rogue/Magneto, some Rogue/Logan. I like stories that expand on the brotherhood and Magnetos thinking. I DETEST stories that turn him into a soft bellied repentant goodie goodie, I guess this applies to hp characters as well. Showing their humanity is one thing, completely changing the character is not.
Link to my delicious bookmarks (i've only recently started saving them so there will be a ton of updates through the end of the year)