I am a veteran of the original Fallout series.
Now that I've finally played Fallout 3 I can say that the complains of the fans are malcontent. Some of the style from the previous Fallouts was hampered but its still very atmospheric. The only things that bother me are minor. Everything else is top-notch(up until now, I'm level 4)
I don't expect the fallout series could have been(realistically) continued any better. I was either a really good commercially successful(slightly consolified, but tastefully unlike Bioshock) game like Fallout 3 or the death of the series altogether. I've read some of the comments on No Mutants Allowed. Although I agree with them most aren't valid issues, if the people that state them played the game beyond the bias I believe they would have a blast.
There are several thing that IMHO
improved over the third installment. Namely the combat system, which reminds me more of System Shock 2 and Deus Ex than of anything else. Oblivion notwithstanding. Which is a good thing. Say what you will about Fallout but the combat wasn't exactly dynamic or, I risk say "tactically" engaging as those titles.
The problem of radiation is something that Fallout 3 expands for the first time in the series. In the originals(probably on purpose as part of the general message) radiation wasn't truly present in a way to make you feel you have to cope with it. In the third installment its a very important aspect as you have to balance between healing wounds and irradiating yourself.
Well that my 2-cents now that I've got to run the game. Which i did on my absolutely ancient gaming rig. That also set me favorably to the game