Prepare to fall in love with Fallout all over again.
I will start by saying that this isn't a revolutionary game. Things haven't changed a great deal from Fallout 3. The controls still function the same, inventory and upgrading are the same. However, what makes the game so great is that everything that we loved is kept the same, while a few nice additions are made. You can upgrade your own weapons now with some tools you find around. The game may arguably be bigger, populated by more factions. And you will have the opportunity to encounter these factions and make them either allies or enemies. A good example is your first encounter with this system, in which you have the opportunity to help a man being hunted down by a gang, as well as help the town harboring him by getting rid of said gang. Helping the town makes enemies of the gang, and the reverse can be done. Or, you can help yourself and stay out of it. The new system, which calculates your notariaty based one who you help or harm, adds more depth to your decisions as they affect the course of the game even more so than Fallout 3.
There are small things here and there, as well, like the addition of some items and weapons to play around with. Fans of the series will enjoy the return of the NCR, and fans of Fallout 3 will enjoy nods to earlier factions, like The Enclave and the Brotherhood of Steel. The story is good and carries the game with a goal of mystery, but there is a bit of a loss of personal connection. Instead of growing up in a vault with your father, and that kind of relationship being the core of the game, you open the game getting shot, having an item stolen from you, and having to track down the culprits. Still, it makes for an addictive mission and you'll no doubt want to continue your trek to solve the mystery. Unlike Fallout 3, however, there are very few surprises. Even the mysterious Mr. House is predictable. But these are minor issues when compared to the fantastic experience of the rest of the game. Also of note are the bugs, which you've no doubt read in many other reviews. However, these aren't so detrimental that they deter from the experience. The PS3 version in perticular suffers from a bit of slowdown and framerate issues. The fact that this game is still amazingly enjoyable is a testament to it's level of quality.
Fallout 3 was an amazing game at the time it arrived, so for a sequel there wouldn't be much new needed. New Vegas rightly keeps everything that worked so well and adds a few tweaks here and there that add to the enjoyment of the game. If you loved Fallout 3, you will no doubt love New Vegas.