Here are my issues with cataclysm...
Revamped old world:
The revamps of the old world in Cataclysm did nothing for end game, and the speed at which people leveled in dungeons meant that most people took the easiest route to level and just leveled in LFD dungeons. gg.
Leveling from 80-85 wasn't as good as WoTLK questing. Vashj was, from all accounts underwhelming and awkward. Hyjal was ok but pretty forgetfull. Deepholm was average. The last two zones uldum & twilight highlands were decent.
The first tier of end game was strong, lots of bosses, a decent difficulty level. Firelands was from all intents and purposes alright with a major cockblock for most guilds on Rag. Dragon Soul is what most people remember because it was out for so long and the constant nerfs just made it bad. Also it felt very anti climactic. Alot of people hated killing deathwings fingernails instead of actually fighting him 1 on 1.
The addition of LFR for alts extended dragon souls playability however it was a flawed system on introduction and because of DS's time out it got pretty stale pretty quickly.
Overall in cataclysm the PvP was generally, a mess, with alot of balancing issues.
I personally deem cataclysm the worst expansion the game has had, and MOP has done alot to fix the issues of cataclysm.