I seem to have a different taste that most here, because I'm a sucker for AMV's that are cute and clever, that is humurous. I know there are others who like that kind too, because otherwise,
IWIWAL wouldn't be so high on the favorites list. Fanservice can be a good laugh when it's made fun of in the right way. Best example of this is, of course,
The Fanservice Video by Zarxrax.
I look for a good song before I look for animes I like. To me, the music and the editing skills of the creator is a lot more important than what anime is used (of course, the anime still has to go well with the song).
I usually cannot stand sentimental videos. In my opinion, the footage very rarely does anything to highten the experience of a sentimental song.
Rock 'n Roll, upbeat, original tunes, and comedy/cleverness is usually my preference. Still, the exceptions are often more interesting than the rule.