Ok, here's what I liked, though some things have left me with mixed feelings:
- the more complex story than in KOTOR 1 (until I realized there's some inconsistency to it and the ends are actually drifting apart instead of meeting)
- no Mission Vao :D
- in-depth interaction with party members (e.g. influence poits, though sometimes it was annoying that you couldn't really avoid losing influence with someone)
- returning to familiar locations (Dantooine, Korriban) - same locations 5 years later actually do bring up some nostalgy
- no cheap Yavin station bonus for the PC version this time (seriously, who was able to afford more than 3-4 items from there?)
- more light/dark path effects on your game, actually motivates to play again (in KOTOR 1 I just reloaded the part where Bastila turns evil and attacks me for the fist time, that's where you select your ending)
- more interesting villains
And here are some things that irritate me:
- "cool new robes"? To be honest, I only look at armor stats, and apart from those jedi armors that required the light armor feat, the robes were rather useless
- money imbalance - vice versa: while in KOTOR 1 you were always short on money (see my Yavin remark), in KOTOR 2 hardly needed any
- I disagree that the game actually motivates you to play a non-combat character, unlike in KOTOR 1 my guardian could accumulate enough pursuade skill to always succeed in it, and also with 10 demolition points and repair for breaking doing items (only the main char's repair skill matters - bug?), you dont really need anything - all other skills can be provided by your companions
- ever heard of DVD-ROMs, Mr. Lucas?
I could tell more KOTOR 1-2 comparisons/improvements. Unfortunately that's all the differences I remember right now, it's been a year since I played KOTOR 1.