Money is the biggest deciding factor of when for me right now, followed by launch games. Last gen I bought all the systems at Japanese launch (American for Xbox), so I blew quite a large amount of money. This time around, I'm living in Japan, so I won't have to pay the huge mark-up prices for importing the systems, unless I want to import a 360 from the States, and I'm still not sure if I want to do that or not.
I'm glad that the 360 is the only system coming out this year; that means buying it and some games in December isn't going to kill me if I want to buy a PS3 next year. But it all depends on the launch line-up.
The only 360 game I really would like to have at launch is DOA4. Not sure about PDZ, and even Ridge Racer is not really impressing me, although I know I'd buy it without hesitation. So I'm trying to decide if it's worth dropping $450-$500 at launch for the system, DOA4, and Ridge Racer.
If I was back home, I'd just pack up my Saturn collection and sell it off, maybe the Dreamcast too, to get the money. Can't do that now; have to buy everything straight out.
Last gen was no better for this killer launch apps either (again- JAPAN, people), and it seems the same way right now. 360 doesn't have its Halo, and there's no way MGS4 is going to be a PS3 launch title. Not even going to bother with Nintendo since they have yet to show any games yet...there's plenty of time to decide on that months down the road from now.