Personally, I'm a fan of the concept of a definitive ending in a Fallout game. Really, that's what totally blew my mind the first time I played Fallout 1 way back in my college years - the thought that the game had actually been keeping track of what I'd been doing (or more accurately - given the illusion that it was keeping tabs on every single little thing,) and then applying those actions to fairly satisfying (and often surprising) consequences, for a nice bit of closure at the end of the game.
Sure, I'd
prefer to have all of that, and just keep on playing, and be able to explore more about all these consequences. But the only game I've known to even
attempt that sort of thing was Mass Effect - and it really takes a
whole other game to be able to accomplish.
To me, the ending wrap-up is one of those things that I feel is distinctly an iconic element of the "Fallout experience." I thought the Fallout 3 end was pretty good,
as a videogame, but less than I was expecting for a
Fallout game. So hopefully Obsidian will be able to live up to the lofty expectations I have for them in this area.

The only qualifier I'd put on that would be regarding DLC - I don't personally like having to reload an earlier save just to play some extra content that I downloaded.
If they come out with New Vegas DLC at some point, I'd hope that they'd come up with a way to expand into a post-game experience.