Josh Sawyer: The central theme of Fallout, which I believe should be present in any/all Fallout games, is humanity's unending thirst for war. The motivations and the participants may change from story to story, but Fallout always comes back to it. Fallout is about retro-future. It isn't the 50s, and it isn't the future -- it's a twisted post-apocalyptic vision of the future as seen through the eyes of the 1950s. Finally, I think that Fallout's dark humor is central to its character. It's a difficult balance, but the blend of grim misery and hilarity is very important.Chris Avellone: The three-pronged quest resolution structure (combat/stealth/speech), the what-people-in-the-50s-envisioned-the-future ambiance (this includes the PipBoy depiction) as Josh said, the free-form environment that lets you wander the wasteland, and the high reactivity based on your character build (skills, stats, gender), your actions, and your appearance/items.”
Fallout: New Vegas is mentioned in the interview a few times, but obviously Chris and Josh weren't able to give any details at this point. However, it has been confirmed that now that Alpha Protocol is finished design-wise, Chris Avellone has joined the New Vegas team as a senior designer. You can read the whole interview here.