Here's a list of reasons. It's not just the first few posts, either. The topic also contain more info later on in the discussion. Can you honestly say this game will not be awesome?

One of you guys better get this game, too, so we can play online together. Free Mode sounds really awesome, but I have a hard time believing it'll be full-city free-roam multiplayer...but if it is, that's going to be totally awesome. I only played Crackdown co-op with Dillon, which was fun, but GTA IV supposedly has SIXTEEN PLAYER support for Free Mode. Supposedly there are no missions, but you can just mess around, and that's all I did in Crackdown anyway. So, why the hell aren't you guys getting GTA IV at launch!?!?