Rock of Ages was initially supposed to have 2D artwork using sprites in the form of “billboards,” which is a technique by which the sprites are always facing the camera. The karts in Mario Kart 64, for example, were constructed using billboards.
Ace Team and Atlus have an interesting developer blog post up about creating the art work for Rock of Ages on the Ace Team forums, that details the decisions that ultimately led to the game being in 3D.
Here’s part of what the problem was with billboards, for instance:
Check out the devblog, linked to above, for another example.