I like to see when two female characters are introduced in canon as romantic rivals, but the slash doesn't use the third party as an intermediate. Basically when the two characters are shown to relate to each other in some deeper way, that doesn't have much to do with how they originally met. There are some Ginny/Cho fics like this in Harry Potter fandom, I remember reading them but can't think of the titles offhand.

One I don't like is a really strong emphasis on femme presentation. Not so much in a neutral way, but when it's contrasted favorably against butchness or gender ambiguity. I much prefer when a character's gender presentation stands on its own, butch femme or otherwise.