I've reached the final level of Durlag's Tower and after exploring some rooms I decided to take a break (and also unload all the phat lewt). I've explored the Carnival area, the Archeological Site area, and I'm about to finish exploring the Lighthouse area. Really, the big issue with BG now becomes much more obvious: very empty open areas with little content to them, yet absolutely filled with trash encounters (I'm tired of wolves, hobgoblins, kobolds, and basically every other trash mob Baldur's Gate loves to throw at you by the fucking dozen). And respawns.
The great thing about Durlag's Tower is that space is limited but useful, and not absurdly claustrophobic to prevent proper navigation (despite the ocassional pathfinding brain fart). The empy areas usually have a gist to them, like traps or surprise encounters. And there's a huge charm to the whole place, with the rooms beautifully crafted as opposed to another woods/grass/mountain/sand pattern. The entire place is great, and I believe Baldur's Gate would have benefited from following in the Fallout design of having random encounters (which ironically are already present in Baldur's Gate, and are the few times where combat isn't so annoying) outside of the main maps. You can easily tell the difference between a handful of petrified adventurers warning you that Basiliks are roaming the area, and Hobgoblins that spawn for no reason a few feet ahead of you.
If Baldur's Gate had followed the design principles behind Durlag's Tower for the entirety of the game, it would have been a much better RPG. As it is, going back to those areas after playing DT feels like serious decline.
I've just finished the Tales of the Sword Coast expansion and I have to agree with you, and likely a lot of other people, Durlag's Tower is like playing a completely different game to the rest of Baldur's Gate 1 and it's expansion. I'd likely be a lot more positive about it if it hadn't been fucked for me at the offset though.
I played BG1 vanilla from a disk and then, when I'd finished that, loaded up the TotSC expansion from another disk. Unfortunately, Bioware shipped the wrong discs to the UK, shipping the USA/Canada version of TotSC which wouldn't play on a UK PC. I had to faff around with a patch to get it to accept the disc and even then the patch assumed I was using the original 6 disc set rather than the collectors edition single disc sets, causing even more faffing around. Then, when I'd finally got it working and had completed the prison Island and went off to Baldur's Gate city to get the sea charts I was able to see that the game hadn't updated to the new TotSC map, Uthgart's (or whatever it was called) vilage wasn't on the map and neither was Durlag's Tower.
I looked around for another patch for this, but none existed. Some google results gave a solution, but nothing downloadable and my game files didn't seem to have the game files they were talking about to amend. After a lot more faffing around I gave up, only to, at the last minute, try a mission start and then it gave me the correct map... but with their preset generic suggested party build, not the characters I'd played BG1 with.
So my appreciation of the game has been marred from the outset by having to learn all these new characters, having no new character creation and basically doing the whole expansion with someone else's party.
In one of your other posts below this one you said that the werewolf island was lazier even than the prison island, to which I'm amazed no-one corrected you, but the prison island is a 100% copy and paste of the Kobold bridge map from the original game. I almost laughed but I was too busy crying at the time. The werewolf island was ok, it fit the same theme as the rest of BG1 with its huge map with nothing in it but the odd NPC, a couple of lame quests and some trash mobs with a half-decent big fight at one specific location. The post-big-battle epilogue fighting was pretty stupid though, as was, like you say, most of the dialogue and character motivation. I'd already guessed the 'plot twist' as I'd tested out killing some annoying NPCs before leaving the village... though just as save, test, reload, not as actual actions.
But Durlag's Tower? Holy shit, they actually made a proper game. I was fully expecting to shit all over it, but no, it's for real. Maybe they were short of staff that week and some guys came over from Black Isle to do some contract work...?
My only real issues with Durlog's Tower stem from, firstly, the disc problem, in that I was now saddled with Edwin as my mage instead of Minsc's girlfriend, meaning I had fuck all Identify abilities, and, secondly, like the main campaign, I was now saddled with an inventory full of runestones instead of scrolls, both of which factors are a real pig if you want to minimise world travel (due to disc instability) and not go back to town before completing the dungeon. So it's hard for me to give a properly fair view on that stage as I was still getting frustrated more than I should have been, but at least it wasn't from bad content or combat. Also, I thought the loot from that area kinda sucked considering the (in-game) hype of that place. When I did finally get back to town and identify everything, only about three things were useful to anyone, and only then on a very scenario-dependent level. A hat which prevents fear? Meh, everyone has anti-fear spells that they'll be casting in that kind of situation anyway. A gorgeous flame sword? Meh, it's only +1 except against XYZ random enemies, so my original +2 is still better unless random scenarios etc. Etc etc etc. A couple of nice +3 items, but the odds of someone using those specific items... meh, if you'd known at the beginning of the game then maybe.
The final, final boss fight for the expansion, the demon, was a cool fight, I enjoyed raging at it until I'd figured it out, but I still think it's an extremely cheap 'difficulty' tactic to introduce "bad guys with powers no-one else has" right at the last minute. I mean, it's hilarious when you beat the guy and get the end-credits-cut-scene only to then die as soon as it over because of the demon's delayed 'fuck-you' spell, but I didn't have a crowd watching me so it was more of a case of *facepalming* until I'd worked out how to spot the new thing and how to deal with it. I mean, it's good because it makes you think, but it's also just cheap because.. it's a cheap dirty trick. So I'm mixed on that encounter. The AI for it was good though.
All in all I'd say the expansion has a lot going for it while at the same time still not really giving me enough to ever likely consider replaying BG1.
I think if I was to ever play BG1 again I'd need some kind of mod. A mod which quite literally cuts out all the crap and just gives a good tight semi-linear adventure. One where someone's gone into each map and just cut everything that's pointless bollocks. Pointless house with three boxes in it? Cut. Pointless swathe of land with nothing it it? Cut. Pointless dead-end strips to dungeon mazes? Cut. I know you can do all that by player choice anyway, but if I replay this in ten years then I'll never remember where all the 'good' stuff is, and I'm never 'wandering about' BG1 again, ever, I know that for a fact.