Crackdown did shit right that GTA4 entirely missed. The whole game is co-op able, right from the beginning. The co-op handles each player's instance separately, so if there's an Agility Orb that one player has, and the other doesn't, it's there for the latter and invisible to the former. You can play start to finish in co-op, whereas GTA4 and Mercenaries and Fable2 only allow co-op play as a guest in the host player's world.
There's nothing roped off to prevent you from wandering into the high-powered area too early, but the platforming is limited so you physically can't get to the spots; smart players realize this and go back later. You can pick up any object and use it as a weapon. Death penalty is minimal, exploration reward maximized.
The two complaints I'd have are (a) no story and (b) shitty soundtrack. The former is ameliorated by them NOT EVEN TRYING to really have a story and the latter is fixed by 360's ability to play any of your own music while playing.
The Agency vehicles evolve as you play. The SUV can CLIMB MOTHERFUCKING WALLS. The sportscar gets machineguns, AND acts as a wedge which flings other cars aside.
In short, let the game be what it is, don't expect it to be GTA3++ or Mercs or anything, and just have some fun, especially with other players. I'll happily drag my copy out and co-op with you; I've still got races and jumps to do.