The truth is Fallout 3 isn't Fallout 1 or Fallout 2, and many of the diehard fans here are fairly angry about it. Couple with the fact that Bethesda did it's normal routine of trying to make too epic of a game, they lost focus and made a pretty good game, but not an excellent one.
When you have played as many quality games as a hardcore gamer since the 70s, you're a bit more critical about the games you play. Yes, Fallout 3 does not live up to the hype. However, that's not to say it's a terrible game. Compared to what has been recently release for the past 2-3 years or so, Fallout 3 is actually an excellent game. Yes, the writing sucks, yes there are bugs and terrible script writing. It could have been alot worse though.
As a computer programmer for the past 15 years or so, I can tell you right now that Fallout 3 was no easy feat. Yes, Bethesda did use the same engine, however it was very obvious that they made heavy modifications to it, and there was alot of work done. Shit hits the fan; it happens. Crap happens, and in a game that is so vast as FO3 is, it's very hard to to catch all of the bugs/glitches/etc. I really wish people would get off this dead horse already. Yes, Todd is an idiot. Yes, they could have done better. However, I'd love to see anyone here even attempt to make something remotely similar to FO3.
That being said, I am not defending FO3 like it's the next coming of Jesus. It's not. It's not an excellent game compared to FO1 or FO2. It's simply above average. However, it's hard to compare a modern game to a classic masterpiece. Comparing FO3 to it's modern peers however, and it looks pretty damn good in comparison.
Edit : On a side note, I would like to add that even though Bethesda pretty much did fail at making a Fallout game, they did succeed at giving us a modern engine that can be easily modified, so that Fallout fans can make their own Fallout game, in their own vision. After playing FO1/FO2 and a great amount of mods, I think it was time for a new engine to use. Sure; the engine isn't perfect, but it is highly versatile and powerful (Just look at some Oblivion mods to see the evidence).