ps. saying "both" is cheating.
What about saying "the right amount of the right kind of complexity"?
I've been thinking a lot about Nethack lately. It derives a huge amount of complexity from a few basic mechanics and a ton of special cases. Drinking from a fountain can summon a nymph to steal your gear, unless you've eliminated nymphs with a blessed scroll of genocide. Some monsters can teleport you, which is a pain unless you have the intrinsic ability to control teleport. Medusa is quite deadly, unless you wear a blindfold. A cockatrice can turn you to stone, but if you kill it and wear gloves, you can swing its corpse around to turn other monsters to stone instead. And if you polymorph into a female cockatrice, you can lay eggs that do the same thing, to throw once you polymorph back! The Nethack term for handling unexpected interactions is "The DevTeam Thinks Of Everything."
I like that kind of complexity, the kind that stays out of your way most of the time and then springs on you all at once when you least expect it. The kind that you can use as building blocks for accomplishing interesting things.
Compared to that, the challenge of clicking each tile of a complexly-shaped vein of magnetite, or selecting which pieces of gneiss to put in this wall, or placing dozens of beds in dozens of 2x2 rooms, or running out of booze because all your barrels are full of roasts, comes up short in terms of contributing to gameplay.
Of course there ARE things in DF that have the good kind of complexity (tantrum spirals, perhaps?), and hopefully it will move more and more in that direction over time, but the "other" kind of complexity can get in the way. Sometimes this is a UI problem, but sometimes it's because the simulation requires the player to do things that don't have interesting consequences other than not-dying (e.g. easy mandates). I think as a general rule, complexity
within a subsystem tends towards tedious micromanagement, whereas complexity
between and among multiple subsystems more often results in satisfying consequences that are unexpected yet plausible.