Rubbish, the problem is that the world that exists to explore is lifeless and boring. Contrary to what you imply, the game's problem is not the lack of a 'guiding' core story or chapter constraints, it is the game's hollow NPC population who have nothing but banal repetetive dialogue to offer, the yawn fest quests and the general lack of anything interesting or non-cliche outside the expansions...
The average Morrowind quest was no worse than your average NWN OC quest. In fact Morrowind had several quests that were very well done, something I can't say for the NWN OC.
While NPC interaction was poorly done and many were clones of eachother, there we're at least a couple NPCs per town that had some unique conversations.
I'll never forget meeting this grizzled seaman in a tavern in the Telvanni Canton of Vivic. He told a tale of some great Nord Sailor-Warrior that perished long ago. The seaman gave me a key that he had discovered that was believed to open a tomb where this guy was buried. He pointed me in the direction of where he found the key and I set off exploring. After about 90 minutes or so, I found a tomb that had a locked door within that this key fit. I opened the door and came to an even lower level that was essentially a Gorgon's Maze. After some time I got to the next level down. Laying in a pond of water was a boat. On it where the remains of the Nord Sailor-Warrior with his shield and axe across his brest and a Daedric Helm on his head. During that adventure I became completely immersed the world and was actually thinking like my character. If that isn't a great CRPG experience I don't know what is.
Morrowind also has a lot of content that you can tell your own story with. In one game I was an insane aldmer who wanted revenge on his Imperial captors. Thus I set off an a campaign of wanton violence and destruction against any and all Imperial authority. I'd cast levitation and fly over Fort Walls and proceed to shoot fireball after fireball at the fleeing Imperial soldiers.
Morrowind really was a unique game. It is most certainly flawed, and you have to use your imagination quite a bit to fill in the gaps but in the end it had something NWN didn't: fun.