Yeah, I should have said 'a drawing medium' rather than just 'a medium'. CG and traditional painting have a lot more in common than pencil/charcoal and pixels.
Cross-stitch is a comparison which has been brought up before on Pixelation. The constraints of the two media are different in nature but similar in degree, IMO.
I guess what I mean , more than just optimization, is the narrowness of choices -- there are many ways to efficiently represent a given human figure, in high-res or traditional art. In pixel art, there are enough that each person's way of doing it can still be unique, but the amount of options is many times smaller, so, for a lack of a better term, the 'face of the system' shows itself. In things like, for example, the limited number of body-plans used for small character sprites, or the small range of views on a tiled landscape. Discreteness, like you say.
Perhaps this view is overly focused on art that will be used in games, but IMO that makes up the majority of pixels.