Oh, man, I loved Lettered, and by the same author, Let's Pretend the War is Over. Brilliant stuff that!
When I first discovered fanfiction I was all about Draco/Harry and I almost gave up on the stuff completely because so much of it was complete and utter crap. Either Draco, or Harry was a total fem in most of them, and don't even get me started on the grammar! *shudders at the memory* I was just about to give up on fanfics when I stumbled upon an awesome piece (that was actually Draco/Hermione) and, well, now here I am.
For me the most important thing is that the story be well written. It doesn't have to be perfect, but it does have to show that the author is making a serious effort and takes pride in their work. Beyond that, equal partners in slash relationships (in Het I'm more forgiving), an attempt at keeping characters as in character as possible, and believable sex scenes. Blatant disregard for cannon irks me. That said, I don't mind a good AU.