Fallout 3 is a smurfing incredible game. Until you play New Vegas and it's like when you leave Midgar in FFVII and realise there's so much more that could be done. The on-the-rails, you-will-fight-the-Enclave-same-ending-for-all Fallout 3 compared to the sheer freedom of New Vegas doesn't stand up well. Fallout 3 is still good though but in terms of writing, gameplay balance and quest mechanics it pales.

The only criticisms I've seen of New Vegas that had any weight are that is buggy. I've had less bugs on that than Fallout 3 after putting in a similar amount of time and from what I know, NV has been significantly patched since release.