I was checking out some of the 'my favorite cel' 
posts and I was really amazed by the diversity.  
I started to wonder what makes a cel really good, 
or at least 'worth it' to everyone else.  

I like cels with lots of details (like three 
color level cels: shadow, highlight, base color) 
vibrant tones, and pretty lines.  I don't always 
like cels from anime I love, I'm usually drawn to 
cels from 'pretty' series like Utena, Shamanic 
Princess, Violinist of Hameln, etc.    I know 
there are a lot of dragon ball fans, but 
personally I can't stand the choppy line style on 
anime like that.

So what does everyone else look for in a cel?