(New post for clarity).
One thing I want to define here is what I'm calling the difference between a rule, a gameplay mechanic, and a gameplay system.
I consider a rule to be a single, atomic description of an event in the game. "The fire attack does 30 base damage. Magic defense is subtracted from spell damage. Being weak to fire doubles the damage." Those are three separate rules that would govern how damage would be calculated if you used some fire attack on the enemy.
A gameplay mechanic is a set of rules and logics and other things that give rise to some sort of... clear and idiosyncratic gameplay feature. In Secret of Mana, you have the charge bar governing when you can attack and if you do a special attack. The rate at which the charage bar charges, how much it can charge, and the like -- those are rules.
A gameplay system is a set of connected gameplay mechanics that work towards a common, larger gameplay... feature. You have a battle system. It's comprised of various mechanics that govern how attacks are carried out, what combo system you have in place, how turn ordering is figured out. And these are governed by smaller rules. And then rules can also govern how different gameplay mechanics interact with eacother.
Gameplay systems tend to be fairly distinct from eachother. Not always, though, and it's especially fuzzy when you've got something like SH:C where every character has their own method of advancement. Are these methods of advancement different gameplay mechanics? Absolutely. But are they concrete systems on their own? Eh, hard to say. Part of it's a question of complexity, I guess. A single-mechanic... system is probably not actually a gameplay system but just a free-standing mechanic.
Anyway though, in that case they're still going to be fairly distinct, so they SORT OF behave like a gameplay system.
The thing is, in any case, gameplay systems tend to be seperate from other gameplay systems. They influence and interact with eachother, generally by altering shared data, like the character stats, but, say, in FFX's battles, you never see the sphere grid pop up for the sake of performing an attack. (Again, SH:C is some oddity here because the... SH:C is an oddity in a lot of ways. Let's just ignore it).
What I see as the difference between WRPGs and JRPGs here isn't so much a difference in complexity but where the complexity is. I seriously need to sleep but here's an example.
In Vagrant Story, the block pushing puzzles, weapon making, and battle system are all very different chains of interactions.
In Gothic 2, all interactions generally all revolve around the same *kind* of interaction.
This is mostly an interface difference but I'll explain how this relates to the actually gameplay mechanics tomorrow.
Ultima games tend to be rather complex
The classic era ultima games -- that is, 4 to 7 -- have really simple gameplay mechanics. 6 and 7, for instance, have all of like, four stats for the character (Strength, wisdom, dexterity, and HP).
Also sort of take issue with the Scumm VM example. That's an interface thing and not actually an issue of gameplay complexity.