While I don't take it that far, I agree with the general sentiment. From a PvE standpoint I think that every expansion has been superior to the last in terms of mechanics. Not for every spec of every class on every encounter, but as a general trend. Say what you will about flavor or pruning or simplification, when it came down to it Blizzard was entirely successful with improving the class and encounter mechanics.
A personal anecdote to this is that I noticed how much harder it was for me to be top DPS in groups. In BC-WotLK I did pretty well for myself and as I played alt after alt I always seemed to be a superior player. But as time has gone on, I've noticed that I can no longer do this. It may be that I've just gotten worse. Or that as the population fell the most dedicated players were those who stayed. Or maybe the best players used to have more to do and it's only recently that I've run into them. I'm not writing this as a proof of anything, but I sincerely think that part of it is that the skill ceiling has gradually gone up and I could no longer compete with those who were better focused on a class and its mechanics.