Had you read my post in its entirety and taken the time to try to understand what I was trying to communicate,
I did read it all. Sadly, I didn't peruse it with the same care and attention I would have taken to a contract. If you write long posts (as do I) then they'll sometimes get skimmed due to a lack of time on the reader's part - mayhaps I misinterpreted your meaning. I thought we were talking about complex and interesting characters, not just alignment. You may wish to also consider the possibility that I did read and try to understand your meaning but it simply wasn't expressed well enough. Your sentence above came across as rude. I'll give you the benefit of the doubt and assume that wasn't your intention.
This is a conversation, there's room for misunderstanding. If I misunderstand something you're trying to say, then just tell me. You didn't respond to all of my points either - that's ok, this is just a conversation.
you would know that I picked Die Hard specifically because it is a highly simple story with simple characters, but that even there the lack of any definite statement by the universe as to the morality of each character leaves the viewer free to form their own opinion
You don't know the alignment of the NPCs in BG. Only the ones that join your party, whose actions are yours to command (with the exception of in-fighting with the odd pairing and the leaving the party if your actions and their desires aren't met) thus making them PCs. So they are also open to interpretation. Is Monty 'evil' just because he's a psycho or does he have a higher calling? I find it strange that you're willing to read motivation into Die Hard but not into BG just because they're labelled as 'evil'/'good'
Who told you Gorion wasn't a bit evil? Who told you that certain people aren't trying to kill you for something other than being evil (this comes across better in BG2 so again, that's a game you might prefer to BG). Who told you that the Flaming Fist mercenary trying to execute a certain person (BG1) is doing his lawful duty?
I get that you don't like the labels, I just disagree that they remove moral questions and motivations. The alignment doesn't make the character, the character makes the alignment.
It may (or may not) interest you to know that some of the creature-files ('creature', here, includes people) weren't given an alignment or were given one in contrast to their in-game actions (this is the subject of a Tweak Mod for those who care about or like such things). To see these, you need to go into an editor - from a gamer's perspective, we just don't know their alignment.
As to mentioning how alignment "makes no difference to the characters or plot - it's just a mechanic to make 'holy/unholy-smite' work and restrict a couple of weapons", I would like to draw your attention to the last part of that sentence. Having alignment foreclose any semblance of being able to form an opinion on ethics in the game is one thing, but now it is actually interfering with and defining the gameplay; I'd hardly write that off as being as minor as you seem to be making it out to be.
That's because you don't like the concept - I rather do. It works as a gameplay mechanic by making certain items usable or not based on your playthrough - thus, there isn't one 'best weapon' in the game for everyone and your class and alignment affect this. If I play through with a Lawful-Good Paladin, I can use that cool dispelling sword, if I play through as a mage, I can use the Staff of Magi. (Class has more to do with item restrictions than alignment - the 'Carsormyr' I mentioned can only be wielded by a Paladin, which makes it Lawful Good by default).
If you want an 'in-world' explanation for 'why' this is - I think (though I'm not sure if this is 'canon') that it comes down to the deities involved. A certain deity, interested in justice, blessed the sword so that it could only be used by one whose heart yearned to use it for justice. Now, it may be that the slaughtering of a whole village was required in the interests of justice (though I can't think of a good example) but if the deity involved disagreed with you then you'd 'fall' as a Paladin and be unable to wield the blade.
Equally, an 'evil' only weapon may have a blood-lust and not allow itself to be wielded by anyone who might 'retire' the blade or decide to use it sparingly (though you'd think any adventurer would be up to his/her eyes in blood anyway so maybe the reasoning is more complex - it wants someone with a similar world-view). (And if you think I'm nuts, talking about what a weapon 'wants' - see BG2's sword Lilacor).
Now, I'm reading more into this than I have before - and it may not be something you like - but your question was 'how can anyone enjoy this?' and my answer is that I don't mind the setting's rules as long as they're consistent. Doesn't mean I'll like every setting, but I do like the Forgotten Realms.
Before anyone rushes to mention that Planescape did the same thing by having alignment specific items
which you enjoyed despite it's 'flaws' in your view.
Don't worry - BG has very few alignment specific items. BG2 also has very few. You can leave them in the dust and not miss out.
As to the characters developing, given that not a one of them can be talked to directly or chime in during a plot-centric dialouge, and that their banter never at any point changes based on our situation, I utterly fail to see how they have any hope in the Hells of growing as characters outside of a hamfisted final dialogue that suddenly creates and ends a character arc.
BG2 has more and there's the BG1 NPC project if you need more of that sort of thing. I didn't use it for my first playthrough but have since then. If you don't enjoy the rest of the gameplay at all though, it's not going to make it for you.
I am utterly baffled at how you seem to think I want every potential twist fed to me
that was in response to "they find Aragorn, he takes them on a journey to clean out a mine that was rumored to be haunted. Unless the story made clear that Aragorn knew that the mines were somehow tied to the Nazgul and Sauron, it would just feel random and there would be an almost audible thud as the driving plot was dropped until it turned out that the mine was being used to ferry orcs to Mordor underground."
I didn't actually think you wanted every twist spelled out - but you're judging characters/plot based on first impressions when they may or may not turn out differently. Again, your example of the man giving to the poor might seem the worst kind of one-dimensional do-gooder until we learn later of his duplicity.
I've not much time left so I'll just add one more little thing:
Icewind Dale had good combat but the story was barely functional (and the quests became "I'm sorry, Mario, the princess is in the other dungeon"), Planescape: Torment is said to have good story (but I didn't get beyond the first level as I found it a bit stupid - I was supposed to be escaping the mortuary and avoiding notice but I was stopping to have a 10-minute conversation with everyone I met - didn't seem all that plausible to me. Then again, that was before I played BG through so I was also getting to grips with the gameplay mechanics and would've preferred a simpler starter-dungeon with a bit of story. Maybe I'll give it another try). BG has a good balance of story, combat and exploration - maybe that's why it's more popular. Not for everyone but good for many.