For starters, I will say that this game cannot be played just once. Even moreso than KOTOR. There are a bazillion choices to make about how your player develops and which plot twists you want to ride on. Having said that, I've only played it through once so far from a goody-two-shoes weapons master perspective, only rarely lapsing to an evil choice (sometimes by mistake) and my light/dark side meter was lit up off the scale. I reached level 26 by the endgame, and don't know what the limit is. Advancement seems rapid until the latter portions of the game. Overall game length is probably about right, but the end portion dragged on a little too long.
Graphics are way better. Can't argue that. For instance, people look a lot more like people and less lumpy where they shouldn't be.
Overall it's more buggy than KOTOR; no patch available yet.The plot is both better and worse. The main plot line is not as good as the first, IMO, but there's much (very much!) more room to wallow in plot variations. Plenty of sub-quests. Some places feel very linear, however, especially around critical plot points like the beginning. (Sometimes you gotta do that, but it shouldn't FEEL like it.) It's a little more obvious, too, that some dialog choices are just different ways of saying the same thing and there's no discernible difference in choosing one over the other.
There are like 10 NPCs running around with you. Your choices can make them (dis)like you more, and when that happens their alignment shifts. One of the characters is Mandalore (actually Canderous) and I got him up to neutral without trying hard. I was playing a male character this first time, and there are at least two female characters to practice your flirting on. I'm not sure how many options a female character would have, but there's at least one obvious target. By the way, there are considerable tensions and distrust between the members of your crew. There are conspiracies and infighting you're only aware of in cut-scenes while you're away. There are at least two characters who are normal to start with, but are force-sensitive and can be turned into Jedi/Sith. BTW, HK droids seem plentiful for a while.
Voice acting is not quite as good, probably a side effect of the larger number of actors and spoken lines. The sound is generally excellent but some musical scores, while well done, get annoying quickly.
Combat has a variety of moves. Some really cool ones, a few just plain stink. In addition to what you remember from KOTOR, there are some additional combat settings. One is a "stance," and you've seen it in other games: a character can be told to be aggressive, use ranged weapons first, support other players, etc. The other is a "force form," and this applies different bonuses or penalties to different aspects of combat (multiple enemies, lightsaber-wielding foes, blaster defense, force strength, etc.) Some forms and powers you can only learn by talking to the right character the right way.
Speaking of powers, there are more of them. I don't recall missing any from KOTOR, but there are plenty of new ones. Some are selectable upon leveling, others are intrinsic to character, and some as I said are learnable in special circumstances.
Lots more feats. That's mostly good.
There's plenty of tribute to KOTOR in KOTOR2. No place or person or droid is exactly the way you remember them, though T3 probably comes closest. Revisit some old stomping grounds (like Dantooine) and see cameos of some of your favorites, including Carth, Bastila, and Malak.
Oh, I left out skills. They play a larger role in KOTOR2. There are times when you are alone, or you're temporarily someone else who's alone, or there's a party branched off from the main group. Since I was a dummy with an arsenal, I didn't have much in the way of skills to play with - but there are lots of items that boost every skill and ability you've got. You're encouraged to put at least one point into everything, so that after boosting you've got a reasonable chance at doing some skill-oriented task.
Workbenches also play a larger role. This time you can craft items, for upgrading items or stimpack stuff. Stuff you don't need and can't sell for squat can be broken down into components or chemicals to make other useful items. And on the topic of items, I have a gripe - there's a few too many copies of "unique" items out there. At one time I had three "Bindo's Bands" and about half a dozen "Exar Kun's Armor." I think they need to make a distinction between rare and unique, y'know?
Oh, one other thing. You shape the past as you play. For example, an NPC might refer to Revan as a man, and you can say "I thought Reven was a woman?" Henceforth, Reven is female in all historical references. Sometimes it's difficult to separate out the "what has happened" from the "what you want" from the alignment-determining questions in a dialog.
*Edit*
Oops, left out the mini-games. Swoop racers now have a JUMP capability. I didn't race much this first time through, but I think you can upgrade your swoop. Didn't play much Pazaak, either, but I think they tweaked the card game so that it's not skewed against you quite so bad. Turret gunning isn't so frequent, and it's not limited to flight. There's one bit very reminiscent of the hasty exit from Mos Eisley in Ep. 4.
