It's not Fallout. It's also not Oblivion w/ guns
There are some people who are die-hard Sean Connery fans. They thought Sean was the best Bond, the ONLY Bond. They never watched another Bond film after him, and you couldn't convince them otherwise.
Similarly, there are hardcore old-school Fallout fans. You could never convince them to get Fallout 3 because they're convinced that all Bethesda made was Oblivion ... with guns. And they're not 100% wrong, either. Nor are they 100% right...
Then there are the people who never watched the older James Bond films. They never read the books either ("OMG! There were Bond books?! Really?!!"). Pretty much the ONLY Bond film they ever saw in their LIFE was Casino Royale. So they judge ALL Bond by that yardstick.
Similarly, there are people whose only introduction to Fallout 3 is Oblivion. They never played the original Fallout games, and don't intend to. Whether it's because they think the graphics are sub-par (which by modern standards they are), or they feel that playing old games is a waste of time (by definition though, ALL gaming is technically a waste of time), they just won't do it. They, of course, think Fallout 3 is brilliant and awesome in every way and you can't convince them otherwise either.
As with many things in life the real truth lies somewhere in the middle. No, Fallout 3 is not Oblivion-with-guns thank God but it's also not really Fallout either. It's some kind of hybrid, stuck in-between worlds and never really pleasing either camp. I guess in the end that was the safest route that Bethesda could've taken.
Final verdict: the game might've actually made me a happier gamer if it wasn't titled "Fallout". As a generic, post-apocalyptic game it wasn't too bad. If it didn't have the Fallout legacy to live up to it would've scored higher for me.
One last comment: the critics were 100% right. The PS3 version is WAY worse-looking than the 360 or PC versions. If you're going to get this game, under NO circumstances get it for PS3, if you have any alternatives at all. Screen tearing, heavy pixellation and anti-aliasing at certain points, and terrible bloom effects are just the tip of the iceberg. Like drugs, just don't do it.