If it's Utena, I'm all over it like white on 
rice!  Er...wait a minute...you meant...

I guess I'll have to go with Pyrrhusgrl's 
assessment and say "nice eyes."  Good balance in 
the cel is a must.  If the character is in a 
weird position or too far to one side of the cel, 
I tend not to go for it.  Hair is next.  It 
definitely has to be long, and bonus points if 
it's pink, and even more bonus points if it's 
curling up in ways that are completely not 
natural.  Snazzy uniforms with shoulder 
decorations...Swords are nice if they're included 
too.  Even better if the character is...oh, hell.

Of course, Nanami is good, too.  And all other 
preferences are overlooked if the character just 
happens to be Touga.  >^_^<