I would say that not only the side quests in DA:O are inferior but also the main quest(s). I was never a hardcore BGII fan, played it 2 times, vanilla and with the Add-On but I still remember Irenicus Dungeon very fondly. There was so much diversity in it, you had the golems, dryads, the windy plane, the rooms of Irenicus girl, the demon at the beginning, the slob in the middle, the dwarves, the thieves ambush at the end etc. You never knew what would await you in the next room.
Compare this to the Sacred Ashes quest. This was just a stupid and mindless grind. "Oh, there is a door/cave, what might lurk behind it? - Ah, some cultist, again, never saw it coming..." At the end I got so bored by all those cultists, I just cloaked Zevran to scout ahead, then I stummbled upon the master cultist, my Party got teleported there and I didn't shed one tear for the 5 same, boring mobs and the nonexistant or crappy loot I missed (Sevran got me something for a Blood-Mage out of a dragon egg and I doubt there would have been anything worthwile if I killed those mobs).
After that, however, it got quite entertaining. That Indiana Jones thing they got going and the Doppelganger fight, if hardly very innovative were fun, all that came before, not.
This was the worst but the same is true to some extent for all the other Main Quests, to many of the same enemies, in the same composition, not enough diversion in between. Especially no little quests involving other people/entities. They appear, like Goodwin in the Mage Tower or Bruck? in the Deep roads, but you only get a little cutscene out of them and are forgotten almost immediately. I didn't even care so much as to read the "Quest completed" entry when I encountered Buck?. Afterwards as I saw a Quest marker on the map back in Orzmarr and went there: Ah, he was lost in the dungeon and I was supposed to look for him, Yeah, great, so exciting. He bought my excessive loot, for that his money was good enough.