Things I like in fanfic?
First of all, it needs at least decent spelling and grammar. If it's above that and includes a good grasp on proper writing style as well, even better. Something that is written flawlessly gets a lot of leeway with me, regardless of actual content. On the other hand, poor diction can ruin the most ingenious of plots.
I absolutely love seeing annoying clichés crushed, especially pertaining to relationship issues like jealousy or misunderstandings that lead to excessive amounts of angst. There are many annoying things out there that keep cropping up like bad money. Seeing such plots dismantled with extreme prejudice is
very rewarding and can make a fic worth reading all by itself.
I like stories that deal with time-travel, time loops and foreknowledge - the possibilities are endless. These are difficult to do well because the author has to sit back and think very carefully about what their characters would do with that potential, forgetting about their precious plotline for a minute. The 'Harry Potter' fandom is full of these, but only a select few are truly well done. In the 'Ranma 1/2' fandom, the only two truly outstanding examples I can think of are
Right Moments by Judah, and Ozz's short story
Exact Change.
I'm also fond of other divergence stories and AUs. The best of those change a single selected fact (or several), and then have the changes propagate through the timeline, the (re-)actions of the characters changing events further. I don't like stories that follow too closely along the lines of canon; for example, I want my 'Ranma 1/2' fics to turn away from what is lovingly referred to as the 'Fiancée Wheel of Destruction'. The Status Quo has gone on long enough in canon - why prolong the torture?
Another theme I like is evil guys being crushed with overwhelming force - sure, a narrow victory works and everyone is safe afterwards, but isn't it far more satisfying to see them utterly destroyed, never seeing their doom coming and spending their last moment wondering 'Just when did I drop the ball this badly?'
Something else that is also very fun to read is to see the bad guys suffering from 'Murphy's Law' for a change. Nothing better than seeing them fail from good old-fashioned, ironic bad luck from time to time. It's a great way to add some hilarity to a story.
More things as I think of them.
Quizer