What I tend to look for, usually subconsciously, are these three things:
Character, Colour, Composition.
I like well-rounded artwork that has multiple aspects I find intriguing. I couldn't just say 'strong emotion', 'Profiles' or 'Pretty girls', but there HAS to be a combination of factors to make me want to own it.
Character: Must be someone I identify with and/or has amazing design. A perfect cel can represent a character's personality but they're hard to come across. When I do find one, I jump on it! Like Jotaro from JoJo's Adventure and Elma from Macross D7.
Colour: I'm drawn towards the combination of Purple and Red, viewing my 'Top 20 page' proves this. In fact, it shocks me to see how often this crops up. I also appreciate the shots that use various shades of a single hue- the monochrome colouring: Kagero from Ninja Scroll; Sen Goku from Cyber City.
Plus, strong lighting is important in acheiving visual impact, like Di Amon from Bastard!!.
Composition: Something a little off-kilter always adds visual interest, like HotIce says. I'm also drawn to pictures that create an informal border to the outside edges. That contained feeling satisfies me.