-A small city with tons of detail especially INTERIOR locations.
-Lots of interaction with NPCs and stores outside of missions. Even stuff like sitting down on a couch, in a chair, etc.
-More RPG and customizability options.
-More quick, fast, arcade style controls. Niko felt like a tank.
-Return of climbing but add diving, swinging off of pipes, etc. Thinks Mirrors Edge meets GTA.
-Destructable environments & lots of NPCs to run over.