Shaping and realism!
A good pair of footpaws look at least somewhat realistic, even if they're toon-styled. The toes are rounded, not square or blocky, the toes all point forward as opposed to splaying outward, and the fur pile points down and forward over the toes.
First, look at the paw of a real cat - for example, here.
Notice all the toes are pointing in the same direction, and that the two outer toes are set further back than the two central toes.
Therefore, the basic shape you'll want when carving out your paws is something like this:
A good method if you're carving the toes from a single piece of foam is to draw the outline straight on to the foam with a black marker and cut vertically straight through the foam so you've got the basic shape of the sole done before you even think of shaping the toes. As far as toe placement goes, you want the 'arc' shape for the front of the paw, but you don't want to place the outer toes too far back, as that makes the feet look birdy and weird!
If you're using the 'add toes to a shoe form' method, there's another trap that some people fall into, and that's splaying. it's a fantastic and quick way to make paws, but you must remember to keep the right paw shape, or the finished paw will look quite odd. The easiest way to make sure your toes don't splay out too much is to use ovals instead of circles for the toe shape. Circles tend to push against each other too much and point outward, giving the finished paw an odd splayed-out look.
EDIT: As some people do like more splayed and circular toes, you can avoid the foot looking unnatural by clever placement of the claws, as in this picture:
Anyway, hope that helps a little!