Subject, meaning behind it or just good techinical skill and if it's a human or animal, the proportions must be somewhat right. If I like a piece I always check out the gallery to see what other the artist has made, after which I almost always end up watching him/her XD
I kind of just favorite whatever I think I'll want to look at again later or that I think deserve it or that I just really really like. But that kind of goes back to the first thing, huh? xD I get put off by a lot of things when I fav though. I'm trying to fav less because I've got 200+ already too.
The subject and technique must appeal to me - it can happen that i fave a drawing i don't find technically perfect though, if i like the style and color. As I'm a color freak, i like striking colorful results (and subjects) and can be relatively loose as far as proportions are concerned. Besides I tend to fave the work of artists i can rely to as a person (same culture) ^
I've passed up faving a piece because it wasn't 'Technically' done well. I.E., a rather beautiful piece, great emotion and color and flow to it, with some anatomical error (disproportionate wings, a wonky shoulder, a pose that shows the muscle in the arm at the wrong angle for the way the arm's held..) that would set me off of helping to 'advertise' it by faving.