Quest cleanup should not be the major focus of an expansion, especially when dungeon finder makes it faster for most people to just skip quest content completely. If they wanted to clean up the questing they could do that without a complete revamp, and as you said most people were already done leveling anyway.
I still don't know what Deathwing was up to, or even why everyone is so scared of him. The only thing he succeeded at was improving the businesses of Stormwind's Masonry guild.
No one really did, they just made it a whole "He's mad at the world so he wants to destroy it" blanket motive. I'm sure they had made a more involved story regarding old gods and such but eh...
I think WoW is ultimately just the "company cash machine" for Activision. They use WoW to make money which they use to fund other projects all the while giving them as close to the minimum budget possible to stay functional. And this time around they tried too hard to appease the newer players and it started to drive their older, more established player base away. What's done is done though, it was a fun ride while it lasted.