I likely can't add anything new that anyone else hasn't already said here or elsewhere. While KotOR is a more complete, traditional game, the things that make TSL special are the characters and the deeper messages/implications about the state of the universe.
Which is why it didn't leave a good first impression with me. I liked it for the first few hours because I was so excited to get into a new story, but as I played on, it lost a bit of its thunder. Throughout most of the game, I felt like nothing was really happening. It felt like I was just going from planet to planet doing completely disconnected things for no real reason. And I don't even want to think about the final act.
As I played through it a few more times and read up on the cut parts of the story, it really gained its own unique appeal, and at this point, I'd say I prefer it to the original.