Thinking of the really big sellers this generation I think I may have an explanation for what sets the mega blockbusters apart from the good seller, and this applies to all consoles.
Looking at the best sellers especially on the Wii and 360 I see one common aspect. Split screen/local multi-player.
Both GoW and Halo have split-screen co-op and multiplayer as well as LAN abilities. Mario Kart Wii, NSMB Wii, SSBB, all these games have some form of local multiplayer. When your friends come over to play you play Halo 3 or Brawl because their one of the few games that still let you sit next to your friend and play.
If you want to make a huge seller you need a game with amazing gameplay, single player, co-op, online multiplayer and split screen.
Unfortunately these are missing in a lot of the PS3 exclusives. Not to say single player games don't sell but it seems to be almost impossible to pull off the mega blockbuster this gen with a single player only game.
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