A plot would of been nice but its still a good game..
Its not a bad thing to stick to a formula that works and I'm glad in a way that KOTOR2 feels so much like the original, however what made the original shine was the compelling story line and sadly the plot, if you can even call it that in KOTOR2 is thin at best, that massively detracts from the gaming experiance.
Still, if you played and enjoyed the first game then go right ahead and get the sequal, you will enjoy it but there isn't as much replayability - the plot being so thin you may not even want to bother playing it through both light and dark, something that was fairly enjoyable in the first game.
There are some minor additions to the game but they're pretty much superficial - including some extra force powers and the new prestige classes - though the prestige classes really have little impact on your character they do allow you to change your character type part way through the game. Personally I think there are only two viable options which is to either focus on lightsaber combat or force powers and the classes that best support that are the dark side's consular/sith lord for force power focus and the light side's guardian/weapon master for light saber focus.