I don't think online gaming is a killer app. It's certainly not been over here; IIRC only about 30-odd percent of UK xbox gamers use online multiplayer (in terms of numbers, but not percentages, there are more PS2 than Xbox online users); the last figures I read had a (flat-lined) 500,000 xbox live subscribers out of about 9m console owners, about 5.5%. (and 780,000 PS2 network adapters sold out of something like 50m+ consoles). And, of course, Segas Dreamcast attempt failed horribly - despite it being marketed as that consoles main attraction.
It's partly dependent on broadband pentration; in the Us it's 'only' around 36% (much lower on other estimations), and lower in many other countries. Not to mention the added cost of a subscription based service on top of broadband costs (my brother has an xbox - I believe his reaction to me when I told him it cost money to play online was '**** off').
Of course there's also the additional issue of actual infrastructure in connecting a broadband modem to a console; I'd wager most people keep consoles in the living room, and any internet stuff with a computer in an office. People will be likely to be put off by paying for unecessary peripherals to facilitate connection in that case.
And a side issue of whether its more fun to play alongside people; I've not used Live, but I've read a lot of horror stories of utter dicks abusing the VOIP service to spew crap. That's something PC gamers might be accustomed to, but perhaps not kid average. Certainly, I would always prefer to play with a mate in the living room rather than some anonymous person halfway across the continent - when I do play online, it's usually with someone I know from real life.
Online also has some side issues; giving developers the 'patch it later' release route (I think there's already been PS2 patches for FFwhatever, as well as a couple for Xbox games), and encouraging overpriced map-packs / downloadable content rather than original development. And of course lag, which can exist even on high speed lines if there's enough packet loss, etc.
AFAIK it's (online console gaming) a very much Amero-centric thing, at least in terms of the actual current adoption. And there's a lot of other countries out there who buy consoles.
To me, online gaming - particularly with a subscription model that only costs more money - is about as important in a buying decision as whether or not the console has 2 or 4 controller ports. And I don't even play 4 player.