I'd say drama more than anything else: a cel is an image in action, and I like cels that show something just as it happens. The joy is doubled when I know the series and episode well, and so I can say exactly what it is that's going on. And if it's a pan or an end key or something else, then I'm even happier.
I've been lucky enough to catch several wishlists, to the point that my posted wishlist is seriously in need of revision. Here's one among many:
http://sensei.rubberslug.com/gallery/inv_info.asp?ItemID=147815
The end key in a sequence in which Meilin (one of my favorite CCS characters) decks The Fight. I gasp every time I see it: color, action, composition, personality, it has it all.
(In fact, it has so much that this scene provoked [i]The New York Times[/i] to run a front-page feature on how violent Japanese anime like [i]Cardcaptors[/i] could have crippling psychological effects on little girls who watched it ... so follow the url at your own risk, kiddos.)