I wish the game was harder all the way up to 90. Not longer, or more of a grind. More difficult.
That's part of why everyone hates the long-ass grind from 1 all the way to cap these days; it's fairly fucking mindless, especially if you're in 5mans. I wish there were options for far more difficult and challenging gameplay, but which rewarded vastly increased levelling speed.
Like, take on this solo or 5-man challenge that's on par with CM 5man difficulty, or the Warlock Green Fire quest, but if you win, you all instantly gain a level, or something.