Yeah, I agree with you, and you know I'm gay for sandbox games. Reviewers complain about the lack of missions, but I think the sandbox formula is arguably improved by not having missions, especially when they are as poorly implemented as GTA's.
Crackdown does things right, and although the difficulty level is slightly borked, it's a good first stab at something new. I do hope we see a sequel. Imagine if they implemented skill trees in this, and different players could have totally different powers? That would make coop excellent.