I am currently playing Neverwinter Nights 2: Storm of Zehir with my brother, and so far I find it impressive how much it changes the game.
The addition of a world map in which you can walk around and find hidden things and stumble on random encounters is nice, but my favorite new thing is the new dialogue system, now every character close to the npc the party is talking to has a chance to talk. If one of the characters has something special that can be said thanks to one of his skills you'll see a small bubble on his portrait, and you can simply change to that character and say whatever he has to say. The player creates his own party of adventurers, but it is still possible to hire henchmen.
Overall it's much closer to the pen&paper experience, and it's something I truly enjoy. The one big downside is that the old NWN2 interface is poor when it comes to controling a full party.
Shikin Haramitsu Daikomyo
"Everything I encounter serves as the perfection of wisdom that leads to enlightenment."