"Because it's MARIO KART you online prejudiced person."
Actually, the quote was:
"Now we're talking killer-ap"
Online will not add to a title's killer-appness. I'm sorry... you could say playability, maybe... functionality, options, whatever... but adding online to a game won't make Joe Gamer rush out and buy it in droves, I don't think.
Edit: You would think that about Halo 2, but I highly doubt online is what sold the game, considering (if I'm not mistaken) that it sold about three times as much as the entire Xbox Live service. No, I think Halo 2 sold for the same reason San Andreas did... massive, massive hype.
Besides, if Halo 2 sold that well because of online, then why did Halo 1 sell?
I'm not convinced.