The Lady In The Tower

So, here is my much-delayed second essay/meta discussing fanfiction in general. I do want to add a disclaimer; none of these are supposed to be guidelines, per se, since I wouldn't want to tell someone how to write their fics. It's just what I've found to be true for me, and also hopefully to start discussion in the comments about what other people have found. So with that, I'm turning to a second topic - the AU, or alternate universe fic.

So, what about you? What kinds of AUs do you like to read, which do you write, and what do you find challenging or fun about doing it?