Fallout New Vegas is Obsidian's follow-up to Bethesda's Fallout 3. This time, the action takes place in the city of Las Vegas, which has managed to stay independent and mostly intact from the war over 200 years ago.
Many of the original team from Black Isle return to the Fallout Universe, and it really shows. Many organizations from the original Fallout games make their return in the glory of the Oblivion engine, catching people up with the West Coast while integrating some aspects of Fallout 3's contributions to the lore.
The complexity of the Fallout Franchise returns as well. No longer are you stuck with a forced story and primitive good/bad karma system, but you have a reputation system with repercussions for having bad rep with different folks.
The story and characters are vastly improved, with time spent getting to know the people around you. Follower sidequests are a highlight.
The free flowing format of the games non-linear storytelling will be daunting for new gamers who are used to being told what they have to do, but this is an old-school RPG in newer clothes. The game challenges you to think for yourself and make less-clear decisions more often, and shouldn't be treated as a first-person shooter above being a true RPG.
The strength of New Vegas is allowing you to really explore the game as a true sandbox. If you don't like the three main factions you can ally yourself with, you have the option of taking control yourself and steering the flow of gameplay.
Mechanically, the game is almost identical to Fallout 3 (which is a good thing). Controls are familiar, but you have a much wider array of customization like weapon mods. Ammo is scarce, so shoot wisely. The Follower mechanic is improved with the Companion Wheel, easing management.
Hardcore mode returns some of the difficulty of old-school games by making you manage sleep, food, and water, as well as giving ammo weight and making your followers mortal.
If there are any low points to New Vegas, the big one is the bugs. There's a lot of bugs and glitches. I can only tell you to save often. The other low point is the aesthetics. Despite being 'new' - the Oblivion graphics are dated, and I'm just not a fan of the character animations.
Bottom line: New Vegas is the real Fallout 3 we should have gotten.