I can sum up most of what influences me in one 
word: DRAMA.  I look for something that "moves" 
when I look at it.  (Not really, though I once 
did have a cel that seemed to change expression 
depending on the color mat I put behind her.)  In 
other words, I like something that's not just 
pretty, but something that captures action from a 
scene that means something to me and to others.

Here's an example:  it's a profile and not well 
centered on the cel, so it's probably not 
valuable in a dealer's sense.  But for me 
it "moves," so it's one of a select NFS cels for 
me.