ARR was probably the hardest expansion, given that the playerbase had only been playing the game for months rather than ten years. If you could go back in time and see your past self play, you'd probably be shocked at how poorly you used to play the game back then. And then you'd realize that you simply became better faster than the game became harder. The Warcraft playerbase was all about 'back in Vanilla' until players could actually re-experience it for themselves.

Anything that a player tries to highlight as 'rotational complexity' really isn't. This game is completely scripted down to the GCD. The challenge is never the execution, because MMOs hands down have the lowest mechanical skill ceiling of any game genre outside of turn-based strategy games. The challenge is the puzzle that you have to solve on each new fight on the optimal sequence of button presses and positioning. And that's where the real fun is.