There's a trade-off between size/scale and graphics I think. The sheer amount of scope and detail in Oblivion/Fallout means the graphics aren't as incredible as some other games, but for me they more than make up for it in exploration, story, discovery, customisation and just the endless hours you can sink into the game.
I've actually been pretty impressed by the graphics so far, it's hardly an ugly game. Far prettier than Fallout for example.
Yeah, the reason I don't want to play Fallout New Vegas is that it's basically the same game as Fallout 3 and Oblivion and the graphics are average so there's nothing to play for really.
If you enjoyed Fallout 3, you'll enjoy New Vegas. The only thing that's the same is the engine/style of the game. It's a new story, new weapons, whole new world. It's still Fallout, but it's not the same as Fallout 3 by any means. It's superior in pretty much every way, aside from graphics.