It's the interaction with the party and the PC. Everything else, (Encounters, kickass abilities, etc...) I can do myself even with a multiplayer party. So I look for the 'new' interactions of an interesting NPC with personality to keep me intrigued and provide the angle of role-play. Without that, you're just playing IWD.
Yep...Your soo right. Intercation with NPC's somehow ads more flavor to the whole rolplaying experience. This is one of the reasons that I didn't like Morrowind(wich still is a preaty good RPG), no party