Fallout: New Vegas is what many believe Fallout 3 should have been from the start. It's a spin-off game developed by Obsidian Entertainment, a company where several creators of the original Fallout worked at the time. Despite having the same graphical engine and the same core gameplay (3D first-person RPG), New Vegas is a very different experience.
Obsidian set the new game in Mojave desert, a region around the Hoover Dam, a crucial strategic point, and New Vegas, one of the few big cities in the Wasteland in all its vice-flavored glory. This setting pleased the fans due to being very close to much-beloved New Reno from Fallout 2. The central conflict introduced several new factions and opened the door for a great number of new interesting quests. The story and characters were universally praised, but that is to be expected of Obsidian.
What about gameplay? It's basically the same as Fallout 3, with a few adjustments that added more comfort to the experience. Updated and enhanced inventory and quick access interface, adjusted VATS and combat mechanics, a bit more integration of perks into gameplay, namely the dialogues - nothing special, but the little things make the world so much more addicting. There is, however, one special thing - the so-called Hardcore mode that makes gameplay more realistic, with fewer ways of healing, replenishing supplies and ammunition, making the experience more like real survival in the Wasteland. It's a lot harder than vanilla gameplay, but all the more impressive for it.
All in all, Fallout: New Vegas is a better, grittier version of Fallout 3 with the flavor of the original duology. It's even more of a cult classic, with all the advantages of the third game and fewer flaws.