Crackdown takes the open world sand box concept of GTA and removes any semblance of story from it. It tries to focus exclusively on mayhem, and the result is a bland game whose objectives lack context and repetitive one-dimensional missions.
The game itself has more dimensions to it, insofar as there are varied side missions, but I find that in general side missions complement the main story campaign of a game, and on that front Crackdown was completely lacking. By the end I found it almost a tedious effort to get to the end of the main campaign so that I could declare the game “completed”. I had plenty of other things to do in the game, but after 15 hours or so playing I really didn’t want to touch it ever again.