NWN is 3E D&D, but it's a bit different, in order to make it work as a computer game.
There's no flying, jumping, swimming, riding or climbing. This disables a few spells and skills, and even character abilities, like the paladin's mount. But classes have been rebalanced with additional abilities, different skills, etc.
Certain feats are out, such as item creation, improved initiative, the mounted feats, and a few others. And a few combat feats, such as Power Attack and Expertise have been changed to make them easier to use.
That said, this is definitely D&D, no mistaking. As soon as I get it, I'll start recreating Keep on the Borderlands as my group's first multiplayer foray into NWN, just as it was my original group's first foray into D&D almost 20 years ago.