Well, first, the way an epic story is told.
It starts somehow mysterious and strange, then develops and grows bigger and bigger. Compare the beginning of NWN2 with the end of MotB and you realize, that a truly epic journey lies behind you. It's not the small "father is slain, revenge him"-story or "evil orcs are attacking the country,defeat them"-story, it 's the "you start and know nothing, and the more you know, the less you know and in the end you are in the middle of a story that will change the earth forever"-story. So far, BG:2 and Plancescape:Torment were the only games that had a similar effect on me, BG:2 with its theme surrounding gods and destiny and Planescape: Torment with its surrounding immortality, guilt, believe, the planes and the ever-aching pain of the unevadeable destiny.
That sort of telling a story, of creating a story is something that is very important and something NWN2 and MotB are able to provide.
Second, the way you are involved into the game.
You have companions. You care for them. You journey the world. You
change the world. You grow, wou travel away from home and you return. You don't feel like you'd be playing some guy in a game, you ARE the guy in them game. You want to know the secrets, that lie beneath the surface (in them game), you want to find out, to search, to develop. You want to become stronger, to get rid of something. You don't play for 30minute to an hour, you play as long as you can, because you get sucked into the game. And after you're through with it, you feel that certain sweet regret, that it is over now.
Third: power. !!!!SPOILER!!!!
It is like that, evolving from the boy on the fields in some village into the plane wandering, god-slaying and fate-changing person is so epic and so wonderful, it's totally different from evolving your character in any other games (besides the two named in your post).
Fourth: Style.
Hard to explain, but I think, you know what I mean. Playing BG:2 and Planescape: Torment is like playing some piece of art. It's an experience. Everything to gether is some sort of...hm...adventure. In NWN2/MotB it's the same. You'll know, when you're through with NWN2.