This is something I've been thinking a lot about with regards to the popular games these days that aren't F2P and I often poke at open world stuff to see what makes them tick. Take away the large scale world and what do you have aside from that? An extremely basic third person controller, a bunch of weapons they probably spent five seconds doing and then a soft body physics simulation for vehicles and vehicle damage.
They seem to spend most of their time spamming art assets all over the places to make the game seem bigger than it really is but if you actually try to 'game' in it you quickly realise that there's fuck all to do and they've put tons of empty space between the game. Seems like people who enjoy these games are the types who are easily distracted by all the shiny stuff and don't really care much about the gameplay of which there is very little.
It's so bad now that when I look at any modern game I check for whether it's linear or not and make sure there's no designing mechanics in there. The reason being is chances are if it's linear the developer probably spent more time with the level design and making it fun than simply scattering assets about the place and overcompensating with procedural generation.
Mechanical complexity doesn't necessarily equate to gameplay depth; Nor does gameplay depth to enjoyment. Some of the most enjoyable games have a really simple gameplay loop - just done well. Great game feel, nice setting, whatever. Take it from someone who's initial forays into game design involved the kitchen sink.
Too many/complex mechanics become cumbersome and usually land up being half baked or poorly integrated. The sweet spot, IMO, is in a relatively small set of polished mechanics that can interact with each other in interesting ways to produce a lot of player options. Options that have gameplay significance, and are fun to use.
Platinum Games are good at this. Metal Gear Rising, Vanquish and Bayonetta are all superficially simple and quick to learn; But there're always layers of complexity to the mechanics which provides plenty of room for mastery - and some of the highest skill ceilings I've seen.