Ratskinner
Adventurer
D&D has generally been a fairly complex, often very complicated, system. It's just the nature of RPGs in general, and class & level based ones, in particular, that they're complex games, and, on top of that, D&D is also 'list based' - you add to it by adding to lists of things, not by re-combining existing elements into new material - so grows in complexity as you expand options. The basic system in 5e is as complex as any d20 game (since it is d20), and thus a bit less complex than the arbitrary/varied sub-systems of classic D&D, 2e included. But that's less complex in the sense that a rhino is lighter than an elephant - it doesn't make the rhino a hummingbird. 5e is far from a rules lite game, and only promises to become more complex as more material is added (one reason the 'slow pace of releases' shouldn't be so discouraging, it means slower growth in complexity).
I think we as a community often fail to differentiate between "complex" and "tedious". There's more than one dimension needed to describe the mental load needed to play these games. So 3e greatly reduced (to my usage) the complexity of the game, but replaced it with a great deal of modifier-generated tedium. 4e decided to keep the tedium, but to extensively re-structure it. This works the other way, too. Fate, IMO, is actually not a very rules-lite game.* It is, however, a very low tedium game, because it avoids (almost entirely) the lists that you mention in favor of free-form descriptors. There is, I think, yet another dimension that would describe the need for at-table spontaneous creativity. IME, Dungeon World is a game that exemplifies that kind of load for the DM. The authors appear aware of it, as well, because the DM advice features notes on slowing the game down and techniques for extracting ideas from your players.
I can't think of another dimension, but I'm open to suggestions:
Complexity: the number of mechanically-distinct subsystems, procedures, currencies, etc.
Tediousness: the amount of in-game accounting and calculation.
Author-load: the amount of on-the-spot creativity required.
Each of these dimensions might be variable as well. Fate, starts with low-tedium, but some groups report play where keeping track of all the scene aspects gets out of hand. Also, as goes without saying anymore, whether any of these are good or bad to any particular degree is in the eye of the beholder.
*To be fair, my standards may be broken by spending too much time reading some very strange storytelling games.