This rhetorical question will probably be misunderstood, but I'll ask it anyway.
How can something that is not Fallout ruin Fallout, or even make it better?
To my mind, that is a very nicely put rethorical question and possibly the best I've read here so far. I do not mean to make a sarcastic statement at all, you are absolutely right: either it's "a real Fallout" or it's not but in case it's not, there is no way it could ruin the Fallout spirit.
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The biggest problem is simply that Fallout was converted to a 3D game and that is something most so called Fallout veterans cannot quite understand. Not because "3D isn't the Fallout style", but because you had to use lots of imagination to play Fallout and Fallout 2. Just take the character sprites for example. All in all there were just a few more than a handful of different sprites. Even the important characters looked like all the others. When you talked to them you would be given a short description sometimes about how the character looked like. This is one of the many things, which made Fallout interesting and unique, but in a way that you often weren't even aware of what was so fascinating. It's something that can hardly be attained in a fully 3D enviroment, but it's not Bethesda's fault at all.
Apart from the crashes and some minor bugs I think Fallout 3 is a [censored] amazing game. It's fun, it definitely spreads a bit of the Fallout spirit, although it's a quite different from the original games. Fallout was unique, but so was Fallout 2 as in contained a lot more humuristic elements (I particularly loved when you put the cursor over a bunch of rubble a few times in a row and it said something like "You do not take your eyes off the stones, because they could possibly attack you"). Many people didn't like the more humoristic way, but I bet most of them are now nagging about Fallout 3 as well, naming Fallout 1 and 2 explicitly to compare these games with Fallout 3, though. Anyway, the humor in Fallout 3 is not as sick and twisted as in its predecessor, but it's funny though, for example the robots which are mumble something against you after talking to you in an over-polite way. It's just a part of the Fallout style and even most of the obligatory nagging videogame-fascists cannot deny that.
Regarding Oblivion, I realized that Bethesda has learned alot from the mistakes they made while creating Oblivion.
BTW, one very important point for me is that I noticed most of the things I really loved in Fallout/Fallout 2 while playing the game for the secong, third, fourth... time. And I have the feeling too that this will also be the case in Fallout 3.
All that is to say I am a Fallout veteran since 1998 (played the first game back then and bought the second after finishing the first about ten times) and I don't think Bethesda has 'ruined' anything. Not even the 'Fallout spirit' like so many claim.
Sorry for the long rant.
EDIT:
Just to tell everyone that I am not a complete "yes man" either: there are things I dislike, for example the lvl 20 'barrier', the 100 skill barrier and some other things like missing perks.