The Broken Hourglass - Artist Interview @ Official Site

RH: When making a sprite for 2D graphics, I can have a very high poly count for my models. For a 3D game, I would have serious restrictions on the poly count.
Animating an object to use as a sprite is actually very easy. It doesn't get hard until the sprite is a human—then it gets hard, because the human brain has an area specifically for analyzing other humans. So a level of realism that would make a tyrannosaurus look okay would still leave a human looking like a cartoon. The slightest error in a human walk cycle looks like a serious limp, yet for a lesser animal it would look quite normal to us.