I think the key aspect of NWN will be its longevity.
Most computer games are played through once and set aside to collect dust. NWN, being primarily an online game, and one wholly-modifiable by the player base, is going to be around for years to come. More tilesets will become available, as well as skills, prestige classes, and expansion packs. In EQ, although the game is massive, replayability is limited to doing specific things you've done before in slightly different ways. For example, you level up your wizard in EQ. You get him to level 60, you join a guild, you kill some dragons, gods, and other uber mobs, collect some special abilities and nice loot, and you're done. Sure, you can level up a new toon, and many do, but the process is the same, the goals are the same, the end rewards are the same. Most EQ players stick around because of the community, even while they acknowledge the flaws in the game, not the least of which is the "been-there-done-that" syndrome common among players.
NWN, on the other hand, will have infinite replayability. As the months and years go on, more and more player content will develop, bolstered by expansion packs from Bioware. Unique and popular Persistent State Worlds will emerge, and so will long-running smaller-scale campaigns. There will be NWN games for every playstyle imaginable...while in a MMORPG your playstyle is practically predetermined. Communities will form both in-game and on messageboards, and players will roam, having their hands in two or even twelve interesting campaigns simultaneously. And why not? They are all different!
We are going to see some amazing, amazing content for NWN. It's going to be around for years, and there will be expansions and a sequel. It will get ported to game consoles. It's going to be huge, far beyond the initial vision of the designers.