They fixed some of the annoying aspects of the first game and added some gameplay elements to spice things up, like reload bonus/penalty and choosing paths.
the first one had both too ^^... or do you mean with "choosing paths" that you can go in a totally different direction? I didn't play the second one yet, but I think it's the same, because the paths are for the co-op mode... player 1 goes left player 2 goes right