AtomicGamer is offering up a brief editorial that highlights some of the elements from Fallout 3 that Obsidian Entertainment will most likely build upon in New Vegas. Of course, there are some aspects that are expected to stay more-or-less the same:
The PC versions of both Oblivion and Fallout 3 were superior to the console ones simply by virtue of modding. Sure, there are a lot of bad mods out there, but there were some superb ones like "Oscuro's Oblivion Overhaul" or the "More Where That Came From" expansion for Galaxy News Radio in FO3. Even if an updated G.E.C.K. editor isn't released, the community will likely still make mods, just with a bit of a slower start, but it'd be nice to possibly see a more in-depth editor that works on some of the original's issues.
The unfortunate part of having a highly detailed world is that more of the console or PC's resources are spent on displaying that than on the characters. This usually means that huge battles with dozens of characters fighting at once isn't really feasible in many games, even if we know that the PC version of Fallout 3 makes it technically possible through mods. Low-spec PCs and the consoles would struggle if this was build directly into the game, though. For this reason, I don't expect to see any change in the scope of fights, but with some interesting new perks and weapons, the whole focus of New Vegas' combat could become drastically different. It's up to Obsidian whether they want to mess with Fallout 3's success in this regard, and whether they think they can improve it.