every halo adds a new dimension to the gameplay. new alien weapons. new vehicles. new big-ass enemies. etc.
resistance 2 has so many new things on it. it has improved weapon design. it has 60 player online. it has a class based coop. it has bigger than screen bosses.
grand theft auto, yes, has become stale. there's no question about that. gran turismo, also the same (but 5 will break the trend because of damage to cars and online racing).
killzone 2, however, as the previews and various gameplay videos have suggested, offers nothing different from part 1 except the graphics. no new weapons. no new enemies. no new colors even.