What about tasteful art design, superb music, frequent use of attribute checks, multiple quest solutions, unique character-based mechanics (Morte´s taunts, Dakkon´s sword, etc.), character alignment for once determined by the actions in-game?
As a side note, and that´s just a semantics issue, I´ve always felt merely saying the game has excellent writing doesn´t sufficiently communicate the high amount of creative/unique ideas poured into it (a lot come with the setting, but that doesn´t matter).
Well, I never finished the game - so I can't speak to what happens later.
The things you mention are highly subjective in terms of their quality - and if you think they stand out to make the game something really special, that's great.
Personally, I didn't think too much of the art/music and stuff like that. That is to say those aspects were done well, but they didn't "stand out" in any way - not to me. Still, I'll grant that the weight given to attributes in terms of dialogue responses was done well. Sort of like Fallout - and that's not a bad thing.
Multiple quest solutions I consider a pretty standard thing in most CRPGs of that era, and I don't recall anything too interesting. I played maybe 50% of the game at one time, and I've retried it 4-5 times in all.
I don't know, it just doesn't do anything for me - but again, that's subjective.
I'm just giving my personal opinion, and I suppose there must be a reason that it's so popular.
I could be wrong that it's primarily the writing - and I'd say more or less ONLY the writing that people remember it for, but since the other aspects didn't appeal to me - I have a hard time accepting those other things being such a big deal to most people.
With all that, I can see how it's simplified to just call it an interactive book. Obviously, that's not entirely fair - because it IS a game that works quite OK as a game, and there's combat and what not. Just not something that excited me at all, when playing it.
I much prefer Baldur's Gate - because I get to develop my own character, and even party if I wish - and I prefer freeform non-linear world exploration, when I can get that.
Beyond that, I just never cared for the Planescape setting.