I feel like, maybe years ago, I learned that NWN was the engine that Bioware used to later make Knights of the Old Republic, but I'm not sure where I learned that or what the engine is called or anything. Assuming that was/is true, then the engine isn't too dated for my tastes, but then my next thought is, "could any of the stories in NWN match KotOR? Because if it has roughly the same engine, it would constantly be reminding me of KotOR and I'd probably be frustrated if the story is lame or minimal or just ... not good.

But I get the impression this has a bunch of loosely related stories, or a main one with add-ons that aren't completely tied in or something? I guess I'm just really not sure about how the content is organized, and if it's any good.

Sorry for the vague question. Or vague non-question. Whichever.

The original NWN1 campaign is quite bad if you compare it to the KOTOR story. When we look however on the Hordes of the Underdark expansion pack, we'll notice that it's not that far behind. You can see BioWare's glimpses of genius there. It's still not as good as Planescape Torment story, but it has a comparable quality as KOTOR. Do you mean that NWN1 story is a bunch of loosely related modules? Technically it has a module construction, but the story is consistent. SoU and HotA are separated from the original campaign (they tell about a different main character), but HotA is a direct continuation of SoU. So the expansion packs are linked.

Also, the premium modules all have completely unrelated stories of their own. From a narrative standpoint, the complete edition might be thought of as a collection of stories, with the tie-in being that all of them are settled in the Forgotten Realms. I am particularly fond of Pirates of the Sword Coast and Darkness over Daggerford.

And don't forget that there are a lot of fan-made modules with stories as good (and sometimes better) as any commercial product. This is where the real strength of the game lies, really. If you rate NWN based on the base game campaign, is a bit like rating paper D&D based on the introductory module in the back of the manual.

I might not be the right one to ask since I made a different experience, coming from Mass Effect and finding KotOR's story rather unimpressive (not even speaking about Planescape: Torment level of story quality), but IMO:

Official story campaigns: Mostly meh (best ones are HotU and for premium modules Darkness Over Daggerford)
Among the free, community created modules: Way superior writing and stories, even compared to KoTOR (e.g. The Prophet series, real page-turner quality with food for thought)

Don't get NWN if you only want to play the official content, get it for the community stuff.

Prophet series is by far my favorite of all NWN community modules and I have played about 50 of the top community modules. Aielund Saga is my 2nd.

I've found Aielund Saga to be rather monotonuous... But I loved every bit about the Prophet.

Baldecaran's modules are like the Troika games to Aielund's Bioware formula: somewhat rough on the edges, but absolutely brilliant if you stick with them. I am so glad I chanced upon the Prophet series after years of hoping for a true successor to Planescape Torment. The Prophet is not trying to be that, but simply tells its own, equally engrossing story in its own setting, and that is precisely what makes it work for me. Fantasy at its very best.