That said, it could just be the capricious standards of mainstream reviewers at fault. It's actually difficult to tell how they will rate titles that are mechanically similar to big-name, AAA-franchises that are showered with "10/10 GOTY, NAY...GOAT!!!!!!!!!!" ratings from the mainstream commentariat. I think zomg has poasted a few times about this strange phenomenon, one that I've recently (well, the past few years) begun to notice. Castlevania: Lords of Shadow is probably the foremost example that comes to mind. It is vastly superior to God of War in practically every meaningful way, yet while God of War is showered in accolades, Lords of Shadow was met with far less praise alongside some spurious criticisms. Why? I haven't the foggiest.
And the situation is none too dissimilar to New Vegas. For everything Fallout 3 was praised for, New Vegas upped the ante (often times greatly so). When other AAA franchises do such, the high scores are guaranteed (hell, even when they don't, the gaming press will cover for them). But not in this one case. And the only salient difference we are privy to is the Metacritic bonus agreement.
Hard to not jump into tinfoil hat territory, no?