Oblivion with guns
It improves on Oblivion in some ways. The environments are more interesting and varied (but most feel more artificial than Morrowind). The faces look normal again (still miles away from e.g. Mass Effect or Crysis). Level scaling seems to be mostly absent. The PIPBoy interface is well done. Physics is improved, and by improved I mean that cups are now bolted to tables and so on.
Fallout 3 is really an action-adventure title and not a RPG. The stats are mostly meaningless.
The set pieces vary from decent to abysmal and do not from a coherent whole. The nature of presentation in previous Fallout games left a lot to the imagination and you could fill the details yourself and pretend it made sense. In F3 everything is available in very graphic detail and it does not come together well.
Radiant AI is still crap. The results are either hilarious or frustrating, depending on your mood.
There's some swearing, prostitution, and tons of gore, but it doesn't feel like it's part of the world, but rather it that it was just tacked on to make the game edgy and mature. This is a rather dubious addition to a game which mostly feels like a Disney production.
They tried hard to include "Fallout features" like being able to put explosives onto people but apparently it didn't occur to them that some were a design flaw in the first place and while it was cute in a 2D isometric RPG game in 1998 it's kind of stupid in a modern fully 3D first-person game.
The Bottom Line
It's Oblivion with guns. It's very far from an ideal "Fallout 3", but depending on what you are expecting it might still be enjoyable. Playing it I can't help but feel that the target audience for Fallout 3 is ten years younger than for the previous Fallout games.