For any competitive games, online is HUGE. For my tastes, this means almost all western games (anything Blizzard, shooters, stuff like Civ). Games like Super Smash Bros Brawl would be played far more if it had decent online. I only wish Mario Kart Wii had a 4p split screen online mode (some sort of team battles maybe).
Anyway, online makes it easy to find opponents for these competitive games. I can't be bothered to bring my friends over all the time for games like Mario Kart or Brawl (or in MK's case, 4p online would be even better). As for my western favs, like RTS and shooters... online is everything. I don't play these games for the single player experience. For MMO's like WoW, for example, it means I don't give a shit about PvE, it's all about the PvP!
