A few things currently.
I won't pay for online gaming. (shush, I won't talk about making sacrifices to pay for peer to peer gaming) I CAN pay for it, I just refuse to.
Cost of the machine, as of right now theres about 4 titles that I NEED an xbox for. If they can get an HDMI box with a tiny hard-drive for updates on the market for 199$ I'd bite :P But that's the most I'll pay for a secondary machine.
As mentioned above, not quite enough exclusives to warrant the purchase. Fable 2 will be a major help in that department... but as it stands now I have little need for one :x
The PS3 is my media player of choice, and as such will be the current center of the entertainment system. But I would like a 360 as a secondary unit (the wii needs more games that interest me :( )
From 0 to KICKASS in .stupid seconds.