I think only incompetent reviewers would call it an expansion.
Otherwise, they'd call Vice City an expansion, Fallout 2 an expansion, etc.
I think you're right about that. New Vegas isn't exactly the first example of a game in the series that's marketed as a new game but that uses the same engine and very similar mechanics. I mean, just look at GTA: Vice City and San Andreas. Their core engine and gameplay mechanics were pretty close to GTA 3, but they also added lots of new things. There were new gameplay mechanics, new vehicles (including new types of vehicles.) A completely new setting, entirely new storylines and missions, new radio stations and music, and so on, and I don't think anyone in their right mind ever called them expanions. Now, reviews probably will point out that the game is fairly close to Fallout 3, but that wouldn't even necessarily be treated as a bad thing. After all, they might say something along the lines of "If you liked Fallout 3, then New Vegas is for you." or something like that, for most critics, probably being similar to Fallout 3 will only be uswd as a point of criticism if they did not like the game.
Most critics will probably praise the improvements it has over Fallout 3 and how great the story is. A few of the hardcoe critics will give it a plus on how it pays more respect to the older Fallout games.
I highly doubt most mainstream critics are concerned with how faithful it is to Fallout 1 and 2, in fact, I'd imagine many of them haven't even played the games and wouldn't know the difference. And I think it's a little early to say they'll praise the story for how good it is when we don't even know if the story is actually good or not.
Oh, but you forgot the critical part that the latter game doesn't have the number 4 in the title, but has the words "New Vegas" instead, which obviously means that it is an expansion and can't possibly be a new game on it's own, despite the fact that everything other than a lot of the game play mechanics are completely different!
Except it doesn't have a 3 in the tital either, if it was an expansion to Fallout 3, it would have been called Fallout 3: New Vegas.
If people think reviews are the almighty god of judgment whether a game is good or not, then the people who miss out on a game because they read a review that the game was "bad" are missing out on the expireience and lack the intellect to judge a game by themselves.
But how else are they going to know a game they haven't played is worth playing? After all, not everyone has infinite money so they can't just buy every game that could possibly appeal to them and decide which ones are good themselves...
Granted, I already plan to play New Vegas because I already know a fair amount about the game, have already played Fallout 3, enjoyed it, and expect to enjoy this as well, but for someone who does not know much about the game and who has not played Fallout 3, they'd probably just pass it by if it did not get good reviews.