If you seriously expected NWN single player mode to come anywhere close replicating PnP D&D, you simply had greater expectations than are reasonable. I've not played the game, and I've not invested too much time in the hype, but from what I've read, the single player mode is just a series of prepackaged adventures that are loosely strung together...now think about it...how interactive can that really be? As a DM for a PnP campaign, could you concieve of creating an entire campaign of, say 10 adventures, completely in advance, including preplanning all NPCs reactions to the party? If you could, you are a far superior DM than I. Hell, I usually have no idea what the second adventure is going to be until we are about 75% through the first!
Now...in multiplayer mode, I see some real potential. The DM could create an adventure and throw in some hooks to future potential adventures. The DM can run the NPCs and when the session is over, ask the players what hook(s) they'd like to follow and create a new adventure. Repeat until you've run an entire campaign.
But even this still has it's limitations. When I DM a PnP game, invariably there are contingencies I haven't planned for. The players do or say something I didn't expect or go off in a direction I didn't intend. In order for me to let them(and I try to always give them a reasonable chance of success for reasonable actions), I have to create stuff on the fly. In PnP D&D, this is usually quite easy...I don't know how powerful the NWN toolset is, but I doubt it will allow you to change/add to the map on the fly or even add in new NPCs.
For instance...I ran an adventure several months ago where the PCs were supposed to explore a cave complex where the main villian and his cohorts were supposedly hiding. In order to add flavor, I told the Druid player that her PC was familliar with the location of the cave. Her clan used it long ago as a place to seek shelter from the harsh winters in the region as well as a place to store food in the summer months. This was before her birth, though so she had never actually been in the cave, but she knew where it was. I did this merely as a vehicle to get the party to the cave quickly without incident as well as help the players feel tied to the setting.
I didn't expect the players to say "Well if she has heard so much about this cave, perhaps she is aware of some 'back' entrance that might not be very well guarded?" I hadn't thought of this...I hadn't mapped the entire cave, just the relevant parts and from my description, it is a large and confusing complex. So it seemed reasonable that the druid would indeed know that a back entrance would exist and might have a good chance to find one. So I had her make a Wilderness Lore check vs. DC 25 (or something high, hoping she wouldn't find it) and she beat it. So I "created" a back entrance on the fly that allowed the party to by pass one encounter and gain an advantage in a second.
I don't see how you could reasonably expect to be able to do that in NWN. So there is no way that NWN can replicate (or even come close) to the PnP experience. While it may use a modified 3e rules set and have some powerful "DM" tools, it is still, at its heart, a Computer Game (probably a really really cool computer game, but a computer game nonetheless). Until they have voice and the ability to allow the DM to quickly create new stuff on the fly, they will not replicate the PnP experience. Period.