I think that the overall idea for F:NV was "It ain't broke, so we won't fix it". The game engine, combat system, and dialogue system didn't change much.
The chief alterations, I think, were:
1. Allowing four final paths to take instead of one, which I think was good. I do think that House and the Legion got a bit short-changed, as most of the "faction" quests seemed to focus either on the NCR or on smaller factions (Boomers, Brotherhood, Folowers, etc.) Each faction would have brought sometimng different to the table:
NCR: Democracy and rule of law, but bureaucracy and high taxes.
Legion: Civilization and total security, but oppression, slavery, and atrocities.
House: Economic growth and technological advancement, but intrusive, controlling government.
Independent: Absolute freedom (to the point of anarchy), but lawlessness and violence.
Choose the one that suits your fancy best.
2. Replacing F3's reliance on Karma with Reputation
This allowed a more realistic gaming experience, and a more complex one, but it seemed to be missing a lot of the heart that Fallout 3 had. a lot of that lay in the player's ultimate goal as well: helping a single faction dominate the others, vesrus helping everyone.