It depends. New Vegas has the same barren feeling that Fallout 3 has. If that's what you didn't like, you probably won't like New Vegas. But beyond that and the engine, nearly everything is different.
New Vegas FEELS like a true RPG. It's a lot more immersive. The story, faction system, and hardcore mode are all miles ahead of anything that was included in Fallout 3. There are a lot of different companions you can get, and they all have their own back stories and "companion quests," kind of like in Mass Effect 2. How you deal with each of them affects your ending. Certain combat skills, such as melee, have been drastically improved to remove monotony. The max level in vanilla is 30 instead of 20. Oh, and it isn't retarded easy or short like Fallout 3.
That's all I can think of off the top of my head.