Ah, KOTOR II. Where to start? First, download The Sith Lords: Restored Content Mod. I state this because the standard game is highly unpolished, is missing a lot of extra dialogue and has many bugs that will frustrate anybody, whether they are new to Star Wars or loved the first KOTOR.
This game shows mechanics that would later be implemented into Mass Effect. A definite meter to determine morality, the breaking down of gear into components (or "omni-gel" for you Spectres), combat that can be paused while issuing commands to you and your squad along with a focus on dialogue options between NPCs and your companions. Your choices in these dialogues could allow you to avoid a combat situation, help progress a quest, or give more information on your companions or your surroundings.
Those familiar with some Star Wars lore (such as the movies or KOTOR I) are probably aware of the black-and-white morality shown. For example, KOTOR I gives you a situation where a woman is attempting to sell a hunters trophy belonging to her late husband. You can either buy the trophy from her (the light option), or convince the woman using the Force to give you the item (the dark option). KOTOR II blurs morality into tones of grey, where the Jedi are not seen entirely as the paragon of all that is good and where the ultimate "light" option is not always perceived as the most prudent one.
Part of this blurring of the lines is due to Kreia, one of the companions accompanying you, the Jedi Exile. A mysterious woman whose becomes more cryptic with every word, her musings and personality have convinced me she is one of the most fleshed-out and well written characters in video game history. The strange this is, not many of the other characters seem as interesting as Kreia in comparison. Actually, I'm somewhat wrong; HK-47 is a great companion as well. His mannerisms about murder, his copies and his past are worth listening to if you want to chuckle. Atton is somewhat interesting wisecracking pilot and the remaining characters aren't worth writing home about. They're not bad characters, just not great ones.
I'm going to stop this review now before I type spoilers and just tell you to play it. If you enjoyed Mass Effect, if you played KOTOR 1, if you enjoy learning about the lore of Star Wars, or if you need an RPG fix sorted out, KOTOR II is a great option. Just make sure to get the Restored Content Mod. It's really necessary to have a good experience.