Crackdown 2 doesn't stray far from the original formula, but rather further refines what was already a great game.
The only real drawbacks here stem from minor annoyances that have inexplicably been carried over from the first game. Occasionally, you will be climbing a building, only to have a very small piece of architecture near the top block your ascent completely, and you will have to find another route. It's not terribly frequent, but it happens just enough to illicit some raised eyebrows. Also, there are so many exploding barrels around that it is almost laughable, until you are standing on a rooftop of an apartment complex next to 9 propane tanks and 4 oil drums, and some random sniper pot shots the explosives and you get sent flying helplessly into a respawn screen. Not OK. There are also a few collision issues here and there, but nothing too bothersome.
In the end, Crackdown 2 is just more Crackdown with a host of minor tweaks that tighten the package, and the addition of drop in drop out 4-player co-op play really boosts the longetivity. While some may be taken aback by the lack of more daring innovations to the core gameplay model, and a couple of little kinks that didn't get ironed out for the sequel, it doesn't really matter. Crackdown was great, and Crackdown 2 follows that game's winnning formula, and the results will have you glued to the screen hunting orbs for hours on end, with or without your friends.