I look for a few things. Character ans series 
come first. Then comes the size of the image. 
Next is the content; what is happening or how a 
character looks. Next is the signifigance of the 
scene. Art quality is obviously a factor. I don't 
like any anime's with _bad_ art (all those old 
things like that old Gundam on Cartoon Network).

So long,
    Larg0

ps. Backgrounds add alot of value to a cel in my 
opinion.