I embrace moral relativism in the real world, but in the D&D world, it is not the case. Good, Evil, chaos, and Law are forces of nature itself. Real forces, like the laws of physics. Gods are aligned to them. Creatures are bounded to it. When you change your alignment you change a part of yourself, the part that is imprinted in your character sheet. Your moral standpoint could determine your profession and class, or even the spells allowed to you. If you are a paladin you lose your powers. There are spells that target alignments, for protection or to harm creatures.
In the real world, you cannot develop a missile that targets only unlawful husbands and evil dictators and leave innocent people unharmed, but in Faerun you can.
From a game perspective, alignment is an RP choice but also a game mechanic, so in D&D is as real and tangible as gravity.
What would be interesting is that you could change the alignment of your characters, like in NWN or P: K games because of your actions, but you have to choose a alignment to start with because spells and class choices depend on it.
Your post illustrates perfectly why alignment change makes a terrific story-telling device that could be framed either as a plot twist or as an extra layer of depth for a particular character. It also has consequences from a gameplay point of view. Others have pointed out that trying to write the story in the form of a journey where you navigate a moral gray zone has been done to death (there is truth to that) so I agree that the writing should be in keeping with the almost palpable presence of Good, Evil, Law and Chaos. Let's say it adds more wiggle room for the writers to come up with more than just a half-baked story for a video game adaptation of DnD.
There's one more thing I just thought of. BG has a few FMV clips which give it a distinct flair and my only wish is that there had been more. An FMV can set the mood, introduce an important character or add weight to a conversation (some will argue BG has too many in-engine cutscenes). I'm not sure they're even called that anymore though. I guess it has taken a back seat to other video encoding formats in the meantime. Of course, how would you go about rendering the player character who stores a lot of parameters upon creation and can potentially yield thousands of different builds? Perhaps a first-person perspective would be helpful, which brings me to my final point. Having lots of throw-away characters makes it hard to establish an immersive narrative and write a well-thought-out story. I recently beat Siege of Dragonspear and while I was fairly impressed with the resolution of the plot, I never tried using any of the new characters. Frankly, I think there are too many companions now. Planescape: Torment has a cult following and to my understanding it does not have a plethora of characters (haven't got very far into the game yet as I'm still on my first IWD2 run).
