Many people worry about the excat same thing. I've been reading around over at No Mutants Allowed, and many "devoted" fans feel the same.
Basically it is mostly in the planning stages, but here are some of the general fears of people.
So to take up your fears:
Many seems to fear that it will be First Person, or maybe Third Person. But not Isometric.
Personally - I could live with Third Person view (I would prefere Isometric), if performed decently. However - after an interview I've seen, I think it would be likely it becaue a First Person view. (But since intiviews are pretty chaotic at the moment, and the statements from the developing team is likewise, there is no hard data yet. We will have to see).
Turn Based: There seem to be a general fear that Turn Based combat will be eleminated in favour of real time. Turn Based is something I think is importent in Fallout, also because turn based directly connects to the Perception ability.
However, with good a autopause, one can mask Real Time as more Turn Based.
I personally want Turn Based the most ... this is Fallout - a roleplaying game - not an action game.
Humor: Also a fear. Morrowind was bland and almost void of humor (except for a few semi-nude Nords that had lost their way). The NPCs didn't have life in Morrowind (and neither in Daggerfall) and were much alike with eachother. Many feel that it is unlikely that they can create the sort of humor that helped make Fallout a great gameexperience. This is based on track record.
One thing that is "scary" also - is that they plan a dual-release for various platforms. This means that the game will be made to (likely) PC and X-Box simultanious.
However - it is early in the process yet, and untill screenshots, diaries and more interviews starts comming out - it is guesswork.
However - I'm currently pleased (glad) that Fallout 3 is comming out - but if Bethseda dosen't hold true to the concept, many Fallout fans will be dissapointed. And this is where my biggest fear comes in - they could make it "Morrowind with Guns"
, because that would attract the rather large Morrowind fan-group, and likely some of the Fallout fangroup and potential yield much money. However - it would seriously scorn the "deeper" Fallout fans, but as we all know - in an industry that is here to make money, the broader the game - the more potential money can be made.