If criminals from GTA worked for law enforcement, they'd be hired by the Agency
One good thing about the game was it's open ended environment. Truthfully you could go straight to the last boss if you wanted. It wouldn't serve much purpose since you would be blown up and shotdown before you had a chance to reach him, but the possiblity does exist.
Crackdown does a good job of allowing you to develop your skills as you progress throughout the game. Honestly there's nothing better than rocketing a group of thugs from 30 feet in the air and than landing amonst their flaming corpse and gather the ammo and skill points they left behind. Honestly I enjoyed my time jumping from rooftop to rooftop while gunning down anything dumb enough to shoot at my but, had the game not been a guaranteed entry into the Halo 3 beta, I don't think it would have gotten as much recognition as it did