That's always been funny, because DA2 had a more complicated combat system. Far more complicated, and better. The mechanics were far superior to DA:O. The encounter design was ****, but DA2 had better documentation, a richer class synergy system, and - apart from the completely insane level scaling - a better way of getting value out of statistics.
In general, I don't think things need to be complicated. Complex, sure, but D&D is complicated rather than complex. There's a lot of gunk you have to shift through to learn that basically a small group of abilities or builds are worth your while, and everything else is a trap build that you should only pick if you want to troll yourself or gimp yourself.
DA:O had the same problem. It's a complicated system, but it's not complex. You can just very easily wander into a garbage build by not quite getting the easily exploitable mechanics.
Ok well I see In Exile just made the same point with a different conclusion. Yes DA2 does actually have a very 'complicated' combat system, but I actually prefer simpler systems as a consequence.
As for D&D, I don't really believe it's "complicated" per se, but once again in many ways the lack of complex elements, i.e. what makes it simpler and more pointed elements are also what make it appealing.
At any rate, this is difficult to talk about in the context of Bioware, because I believe "complexity" has been the norm for pretty much all their games, the two foremost exceptions being BG1/BG2 and NWN OC (where you might, as Exile suggested, find the small group of powerful abilities)
I don't really think those abilities are really hidden though, great cleave, devastating criticals, epic wizard spells, etc. I'd even say that was the appeal of D&D to me originally was with many RPGs i.e. those more descended from roguelikes or great minutae you had you know the big "fireball" sort of spell there that was just screaming to be taken.
Plus lots of abilities in D&D are very clearly indicated (Magic Missle is the attack spell, armor is your defensive, spell, etc)
HotU and ToB respectively went in the "complex" direction, introducing wealths of things like more prestige classes and all that to the proceedings...
I think the big twisting factor is this perception of D&D with it's dice rolling and large rulebooks and all that as the center of "complexity" when in reality I'd say it's in many ways a center of simplification (and part of why it's good)
Sure, it probably had some excessive elements, which is why as stated Planescape Torment represented a really good synthesis it seemed.