I just watch Danger Dolan's video on why Cataclysm had so many issues.
I agree with a lot of what DD mentions, the monotonous grind to get gear. The requirements to do dailies up dailies to get anywhere. I also agree with the lack of flow leveling between zones. You had 5 distinctly different questing areas and no real tie between them other than 'Deathwing broke this.'
As of note, I wasn't one of the 'casuals complaining about difficulty'. I totally expected the new content to be harder. Until my gear outstrips the content (like at the end of an expansion), repair costs are going to be high.
Sadly I never did see a lot of the Cata raid content. LFR was there, but not as prevalent and when it was I was heavy into PVP.