Atomic Gamer has put up an article called "New Vegas: How to Build on Fallout 3." It's a good read...here are the highlights:
So now, the roles are reversed in the community; Bethesda fans are unsure of Obsidian, while the old-school Fallout fans, who generally disliked the third game, are happy with the decision. It's hard to take sides on this matter, but I think that if we look at how LucasArts basically crippled Obsidian's development of KOTOR2 by giving them only a year to complete it, we can forgive them a bit. To me, it's a pretty major feat to get even close to completing an epic RPG in only 12 months, and the fact that they got it good enough to get generally B-ish scores from most review sites shows that Obsidian got really close to their lofty goal.
We've seen some reveals of information in recent magazines, and it's clear that Obsidian is not drastically changing the formula that Bethesda laid out. It's still a first/third-person shooter in real-time, and it reuses at least some assets from Bethesda's game. We've seen comments on what Obsidian is doing to improve the dialogue system while still working inside the boundaries of having to have voice acting for every word.
The first/third-person action will return for sure, and it's not really feasible to try and vastly overhaul a successful feature like VATS. Simply put, everyone involved in making this game still wants to sell millions of copies, and this is the exact kind of game right now that can do that. As Mass Effect 2 has shown, you can have an action-RPG that's very heavy on dialogue and story and still sell millions, and that seems to be Obsidian's aim. If the lion's share of their work is on the storytelling, dialogue, world-building, and atmosphere, then they're in good shape.
Conversations can be improved with more natural choices for your character's lines, and the ones that the Vault Dweller had in Fallout 3 didn't always make sense. The craziness of the characters was also a big deal, but if Obsidian can stay away from generic stereotypes, they will have a more well-rounded cast. So we need fewer characters like Dukov or Megaton's Lucas Simms, and more characters like Mister Burke or President Eden.
The author goes on to talk about all of the problems he saw in Fallout 3, and how Obsidian can fix them in New Vegas. Head over here to read the whole article.