Castlevania, Enslaved: These two really surprised me, especially Castlevania. I thought they would have sold significantly more than they did.
Split/Second, Blur: These are hardly "great games", and they're in the arcade racing genre, which has never really been all that big.
Alan Wake: The definition of niche.
ModNation Racers: Middle of the year release, nobody really knew what it was.
Red Steel 2: People disillusioned with the first game in the franchise
Spider-Man: Shattered Dimensions: People disillusioned with the last thousand games in the franchise.
Prince of Persia: TFS: People suck; also, lackluster reviews (though not from gamrReview, I'm glad to say, since I'm the one who wrote that one)