The game does everything the original did at least a little better, and the addicting multiplayer is icing on the cake.
First of all, if you've played through the original Uncharted, (I have, I just don't own it... probably will buy it for the hell of it though) then you pretty much know how the sequel works. There are no earth-shattering changes in gameplay or anything else for that matter, only small improvements over the first one. As the original was already quite amazing in pretty much every category, this is not a bad thing at all, and the sequel lessens the very few annoyances the original had, such as the fact that sixaxis aiming for grenades is now optional. Graphics are also somehow even better than in the first game, and epic cinematic events are now very well integrated into gameplay to increase the sense of immersion. Music and sound effects are great as well, really pushing the idea that this is an epic blockbuster game that rivals (I think surpasses) films. But what really impressed the most in the first game was the story and the great sense of characterization. The characters were truly formed, with clear personalities and motivations. Uncharted 2 is no different, and is perhaps more compelling after having played through the original, as you get to see how recurring characters act, although there aren't many if any direct references to the original.
I must certainly address the multiplayer aspect of the game. Let me start by saying that from the time I heard the game would have multiplayer modes, I really didn't care too much; I looked forward to the game purely for the single player story aspect, as I suspect most others did. I didn't really worry that the single player story would suffer at all from the resources spent to create a multiplayer mode, but it just didn't matter to me. In fact, when I beat the singleplayer mode, I almost forgot about the multiplayer. But I am glad I didn't: as I had already somewhat learned from the multiplayer beta released before the game came out, the multiplayer mode is fun and quite addicting. In a nutshell, I would describe it as the great 3rd person action of the single player mode, mixed with aspects of both Call of Duty and Halo's multiplayer. Uncharted 2 has "boosters" which function pretty much the same way as COD's perks, and it also has video playback of games played just like in Halo 3. The ability to continuously level up and get new boosters, characters, and other things is what makes the multiplayer addicting, in the same way that COD is addicting. There are many adversarial modes as well as 3 player co-op modes. I haven't played too much of the latter, but I see myself playing the multiplayer mode for quite some time; I am determined to get to the maximum rank! (I'm not sure what that is, but I'm sure I have a long ways to go!)
I won't bother going into the specifics of the game beyond that; you don't really need me to tell you this game is good: as of this writing, metacritic's rating for Uncharted 2 is a 97, so clearly many know this sequel is a sure bet. For this game, I think "more of the same" is almost the best compliment it could get. With the addition of multiplayer, however, that statement would have to be revised to "more of the same, plus more".