Rock- Dealing with the mess that was old-world, 1-60, leveling. No one can seriously argue that the 1-60 experience wasn't a relic and wasn't the worst part of the game by the end of Wrath.

Hard Place- Taking the time to revamp 60 levels of content, while also providing significant endgame content, all in a realistic amount of time.

I'm going to go out and say that this "early burnout" was, for the most part, unavoidable in a lot of respects. There simply isn't nearly as much endgame (80+) content in this expansion, and that's because the majority of the time spent was on the 1-60.
Not counting the 4 new low lvl zones that came with two new races in BC, endgame players have received 99% of the new content in the last two expansions, and that system was simply not sustainable.

As far as the difficulty goes, I honestly think a lot of people are overreacting, I know I sure did.

Sure, those first couple heroics were brutal, but it really didn't take long for people to get the hang of them, I pug heroics all the time and I would say I'm 85-90% successful. Really my only gripe with them is that they are too bloody long.
Raids were the same way, my very casual (1 night a week, didn't even kill LK) guild spent two weeks on Magmaw beating our heads against a wall, convinced that Cata raiding wasn't for us, yet here we are a month later 5/12, a handful of us still with below 346 gear in some slots.