[quote name='MSUHitman']Can't the game be played off the disc? Or does it just have the 22GB install if you choose to install it? If it MUST have the install to play AT ALL, 50-75% of 360 owners out there won't be able to play it as the vast majority of 360 users are still with the 20GB HDD.[/QUOTE]
I think installing on the HD is optional but recommended for optimum performance. Here's an article on geek.com:
"360 owners are in for a bit of a shock when they come to install the game, though. John Carmack has confirmed that if you choose to install, then expect to see 22GB of space disappear on your hard drive. Not only is that significantly larger than all other games as far as we know, it’s going to take a while to transfer across.
It’s probably not worth considering bypassing the installation as Carmack stated in his QuakeCon keynote that for the 360, installation is “the way to go.” So in the long run, a painfully long installation is worth the wait to help cut down on much longer load times that will end up costing you even more waiting time."