Really a lot more depth in the world and just overall better. Obsidian has taken a decent game and made it great. Take fallout 3, expand it, add more diversity, add more interesting locations, triple the amount of factions, add a better theme, and add more immersion and you have fallout NV. Take fallout 3, expand it, add more diversity, add more interesting locations, triple the amount of 27 hours in, just getting started.