I cannot even begin to describe everything in the TeamXbox exclusive story here. To start with, Crackdown will be getting three separate updates - a title update, a free content pack, and a premium content pack. Boy, the developers weren't kidding when they said they had content coming up for the game.
The free content alone has a ton of awesome stuff thrown in, the most interesting would have to be the "Keys to the City", which more or less enables every cheat imaginable in the game. You can spawn any Agency vehicle (including the new vehicles in the content pack) anywhere you are in the game world, you can speed up your ability learning, you can set your skills however you want, and of course unlimited ammo! All of these can be enabled/disabled on the fly in a nice pop up menu. This can also be done on a "clean-slate" game so you don't mess up your legit game. But that's not all for the free stuff. There will also be vehicle impounding and some sort of trophy show off sequences with your gamertag and all.
The premium content, well where to start. Three new vehicles, a buggy, a new pursuit vehicle and an armored vehicle of some sort. Six all new street races where you can race with up to 11 other opponents, including one other person over XBL. New weapons! These include the "MSK Lobber" which burps out mini-rockets that will swarm once locked on target. A new proximity charge, a harpoon weapon (that you can impale enemies with, attach them to cars/buildings etc) a cloaking device, and last but not least, a new mini-gun!
There will also be new game types with the premium content pack: Max-Agility Roof-Top Races, where you can race against up to two other people over XBL in nine brand new races. Then there is the Stockpile mode, which is either a solo or co-op mode, where you race against time to get every single orb included in this game mode. You can work together on this one as well. Leaderboards included, best times get uploaded. Then there is Checkpoint Chase, which is a two player mode and can only be started from the Agency garage. A random target is chosen, and shown on your tracker (but not the map) and whoever gets to it first gets the point. This then activates the next checkpoint, and so on and so on until there is a winner at 10 points. Rocket Tag is the next game mode, and this is more or less a game of Hunt and Hunted. The Hunter has the straight shooting hotshot rocket launcher, while the Hunted has a crappy pistol. The Hunted must survive for 60 seconds to win.
There will also be achievements naturally. In fact, the game will now have 50 in total. And the content will add 350 more gamerscore points should you unlock them all. Whew, that's about it for me. Read the full story at TeamXbox for all the details on this amazing content for Crackdown.
*Edit - I added the Major Nelson link, as he provides another huge amount of information on this story. Enjoy.