The way I characterize the second kind of AU is "let's kick canon in the knees at a carefully chosen point (or two) and watch the changes spiral out." :-) Those stories do tend to be more interesting to me... though that is also because I find that at least half the interest of "let's drop these characters into a different setting" lies in having an interesting setting, and sadly, a lot of those stories have settings as interesting as mud and as thin as tissue paper. :-(

Also, I tend to feel that the characters should be different in a different milieu and their plot should consequently have at least some variations, but I think that's the character-driven aspect cropping up again. (Well, and also my world-building obsession, but that goes without saying. *wry*)