When you have more than one unit in the place of one, its not possible on a 3d game so I hear, but since sc is 2d we can "stack" units on top of each other.
I don't know if I like the Zerg enough in SC orig. But I don't like the direction the Protoss are going in SC2, so I will probably switch then. I like the way the Zerg are turning out from what I've seen so far.