Export: I don't think all the people, yourself being one of them, that say NV is a big improvement over F3 are just talking out of their asses or are under some sort of illusion, but I really can't see any improvement in any aspect. I still can't remember a single character or place name. It's all totally vapid to me, just like F3. I'm earnestly asking someone to explain this to me because I do want to enjoy NV somehow. I mean, some examples or characters or subplots that are this much-vaunted "good writing" would be welcome - it may just be that I've got to get to a certain point where it all picks up.
It's more of an RPG, basically.
1) Better writing... some of the dialogue and characters in New Vegas stands with the best in the medium.
2) Factions you can annoy or please, mutually exclusive toward the end, causes real choice and consequence when doing missions.
3) More limited perks and stat improvements mean a more defined character.
4) A more "real" place, rather than a theme park. You feel like you are in an area where many factions are on the brink of war and you can decide the outcome. In Fallout 3 you were gonna fight the Enclave and side with the Brotherhood in every way that mattered because the game forced you to.
5) Hardcore mode offers even more RPG feel.
There are other reasons, but those are the core ones for me. I started RPGing with the original Fallout and other mid-to-late 90's RPGs that emphasized freedom, choice and consequence. New Vegas lives up to that tradition in a way Fallout 3 never even tried and that pleases me. If you don't notice the difference though perhaps you're just not into the same RPG elements I am.