In fact, the only serious failing I can see is that Fallout 3 still doesn't go much beyond Oblivion in creating a convincing world. There's still something a bit flat or odd about the character modelling and animation, not to mention the presentation of dialogue scenes - all areas that Mass Effect got spot-on right - and there are still occasions when the behaviour of the non-player characters doesn't seem to tally with your actions. Sure, charm skills and the wider system of karma ensure that you'll get different responses depending on your charisma and your deeds, good or bad, but wouldn't it be good if your fellow patrons raised an eyebrow when you shot a man dead and stole his clothes in the local bar? Or if the body had been moved when you returned a day later? There are times when you'll feel a real emotional connection to the population and their stories, but there are also other times when it all feels fake. Played over the long haul, Fallout 3 succeeds more than it fails, but there are still key areas where Bethesda could improve.
That said, this is an incredible game and easily the best RPG experience I've had since The Shivering Isles. Epic and majestic, Fallout 3 is going to disappoint anyone who comes in expecting a thinking man's action game, but in a market saturated with action games, thoughtful or otherwise, it's more important that it satisfies those looking for a deep and rich adventure. Many games get less impressive the more you play them, as the designers run out of ideas or the story just runs out of steam. Fallout 3 just seems to grow and grow. In fact, there's really only one good reason not to buy Bethesda's RPG: if you get it now, before Christmas, where on earth will you find the time or inclination to play anything else?
Remember it's a marathon, not a sprint, and Fallout 3 will open up its riches. While it's not as immediately brilliant as Oblivion or Bioshock, it counters this with a strong atmosphere, an epic tale and deeper game mechanics than either.