This is a topic I've pondered for a long time. I'm a long-time lurker here and rarely ever post, but this topic in particular is important to me because I really feel like Cataclysm is where things all started going downhill, and not necessarily because of game systems like LFR.
The biggest issue I have with Cataclysm is the 1-60 leveling revamp. At the time I thought it was great, mainly because I got a new character from 1-60 over the course of the Thanksgiving weekend immediately after the revamp was released. However, as time has passed I realized that the 1-60 zones are ultimately poorly written at best (awful referential humor) and poorly planned resulting in immersion breaks at worst (Garrosh in Stonetalon, Westfall perpetually under siege, etc.). Cataclysm seemed to start the trend of continuity-breaking the game experience with each expansion which has never been fixed and continues to get worse. It just makes leveling a new character so jarring. As a veteran player, the lack of attention to detail / forethought tells me that Blizzard has never really had clear plans or doesn't care about the game's future/story, and a new player must be utterly confused about what in the hell is going on.
Ultimately, I feel that the Cataclysm induced continuity breaks ruined the mystique that the world had and cannot recover without a carefully thought out massive (like 1-max tbh) questing revamp which will likely never come short of "WoW 2".