What makes an RPG good?

[kyoryu]

I'm doubtful of that, though it might depends on what you call "complexity".

A lot of peoples like the depth that complexity allows.
But I don't know anybody who would prefer a D&D in which every ability score is a multiple of 0.73 instead of an integer [and the modifier associated is ((N*1.37)-10)/2, rounded down, so same game balance] just for the sake of complexity.

Complexity is the cost of interesting features of a game, but it's still a cost and you don't want to overpay.

So, look at 3.x. Sure you can make a completely non-combat character by searching through 20 splats and adding stuff together.

Or I can do the same thing in Fate or GURPS with a lot less effort.

Some people like showing the mastery that they can do that in 3.x They like mastering the complex system and showing what they can do. It's a thing in tabletop games, in video games, all over the place. Like a lot of systems offer "you can do anything" but, really, boil down to a half dozen builds. People like the complexity of figuring these builds out, rather than just being handed that half dozen choices, even though in terms of real depth it's basically the same.