I think you are being a little bit unfair here. You mean those spells and trashmobs did not make combat repetetive in BG2? You could use exact same spell combinations to win nearly every fight. True, there was no aggro mechanic, but it means your enemies were even dumber - they attacked this first character they targeted upon - whether it was the weakest or the strongest, and it was easy to kite them (in comparison in DA:O there's always at least one enemy engaging each of your characters).
BG2's fights were shorter and there was a huge amount of variety in every dungeon. DA:O has very few enemy types that can be fought in a similar way. Most of the time you're fighting generic melee units, ranged units and mages, and it doesn't make that big a difference whether the melee unit is a darkspawn or a werewolf, or whether the mage is a darkspawn or a blood mage.
BG2's AI wasn't great, but the encounter design was miles ahead of DA:O, different enemies had different spell resistances and saving throws as well as special abilities, there were mages with protective spells, there was a big difference between fighting an illusion mage and a destruction mage and so on. Unless you rested after every fight (which was unfortunately possible), you'd run out of your best spells really fast whereas in DA:O you can spam them over and over again in every fight thanks to cooldowns. DA:O's AI may be better, but there's just nothing that would make the fights interesting. Pretty much all of the darkspawn fights feel the same and there's like 50 hours of that in the game. Dragon fights should be the highlights of the game, but in practice you're just doing the same thing for fifteen minutes until the damn thing dies. Same thing with revenants and pretty much every other tougher enemy in the game. Save for a few good fights, it's either trash mobs or a ridiculous HP bloat. Both are boring and repetitive.
Tank-healer-mage? That's BG2 in a nuttshell. Actually DA:O gave warriors something to do on the battlefield other then being there and tanking.
BG2 had a party of six and the healer could actually be the tank. I don't see how tank-healer-mage could possibly be used to describe BG2. On the other hand DA:O has only a party of four which leaves a lot less room for different party compositions, especially since there's no multiclassing and there are only, what, two mages in the entire game? I played a rogue, so basically the only party composition that made sense for me was rogue-warrior-mage plus an additional warrior or mage, and the latter was always clearly the superior choice.
Also, what exactly can DA:O's warriors do aside from tanking? Based on their talents, the warriors' role is clearly to draw enemies into attacking them, i.e. being tanks. The talents mostly include minor buffs and special attacks, and there's a bunch of talents that are built specifically around the aggro mechanic (Taunt, Grievous Insult, Disengage, Threaten). Many fights become just battles of attrition when you watch those health bars slowly drain away while Wynne casts another healing spell on your tank every time she has one available. Since the system is based on cooldowns, it's easy to just repeat the same pattern over and over again in every fight until you win. BG2 was nowhere near being that formulaic.
The only advantage BG2 had was that it had more monsters and spells, but there were two reasons for that:
a) BG2 could take advantage of huge amount of AD&D source materials
b) BG2 came after BG1 which didn't have very great monster variety. At the end of the day Ogres were just Ogrlins with better stats and a different character model. That's not much of a difference, gameplaywise. Now Ogres in DA:O had their different sets of abilities - they weren't just biffed Hurloks, so fighting them was at least a little bit different. In other words, DA:O could have expanded in the sequel. But it didn't.
I agree that comparing DA:O to the first Baldur's Gate would be more reasonable. It's obvious that DA:O couldn't have the same amount of spells and monsters as BG2, and it would be unfair to expect that. However, I still think that DA:O's core mechanics are deeply flawed and even major tweaking wouldn't fix them.