I have to disagree with you on this one Gallifrey, BG2 is indeed considered brilliant by most people, myself included. A lot of people become overwelmed by the high level combat, especially the magic system with nearly 200 spells, but I find it to be a thing of beauty once mastered.
The problem I have with the magic system is really two-fold. One, I've never, ever, ever in all my years of D&D gotten the hang of magic. It's never appealed to me as it is contradictory to the way in which I play. Second, in BG2, the magic becomes a core mechanic to the game, and everything else is largely secondary and even tertiary. If you love D&D magic, then BG2 will be the best thing ever. If you don't like D&D magic, it's a millstone around the neck of the game.
The only drawback, which I can readily understand is a turnoff to many, is the learning curve of the high level combat. Many people simply become frustrated and end up quitting before they realize what a magnificent game it truly is.
Well, I did finish SOA, but got utterly fed up in disgust with TOB. Not that it was hard, but when every stock enemy I killed was dropping +5 weapons and I could have Viconia dual wielding with magnificent effectiveness without any skill in that combat style, I just lost what shreds of interest I had left. It was too over the top and without any story or character interest at all.
The high level combat isn't hard, it's rather the exact opposite, I find.
I accuse D&D high-level system rather than Bioware. To my eyes, D&D stopped being a role-playing system but a power gamer generator, of which even Gygax complains. I became more interested in other systems quickly and I see the reason more clearly as the time passes. Considering that, I have to admit that Bioware did rather well. At least about the combat system.
In regards to BG2 it's a bit of both. BG2 was built on AD&D, but and I never found AD&D to be particularly geared towards power-gaming. Sure, it could be, as BG2 showed, but in itself it always seemed more moderate. It was BioWare that took it to extremes, really. They tossed in the extremely powerful equipment and then had to toss in extremely powerful creatures to counter that (or vice versa, whichever) and it became a replicating disaster.
BG2 was also made at the time that 3rd Edition came out, so BioWare cobbled in some vaguely 3E-inspired features like High Level Abilities, thus making the system a bit of a hybrid which threw the balance of more.