With my 360, it was to game on XBL with my bro, who had one already, so I suppose I'd have to say XBL was the killer app. We don't live far apart, but our whacky schedules make it easier to not have to go to the others house most the time.
I'll be getting a PS3 this week, tried to get one the other day, but the Viera, Fl. UberTarget was actually sold out! :Q The killer app I'm after is the Blu-Ray player. Saving $60 and getting the 5 movies was the clincher. The trimmings e.g. 3rd party HDDs, linux, and initial customer satisfaction with reliability don't hurt either. I've been wanting to play Resistance for a good while now too.
A Wii may end up in my son's room, and the killer app is the Kid friendly game library+ interactive controller.