I often say WoTLK is where Blizz shot themselves in the foot.
But Cata is where Blizz started to line their coffin.
Turns out the LFG system we loved from WoTLK was a disaster when it came to new content. We also wanted harder dungeons, which we got, but Wrath Baby Syndrome + LFG = Disaster for pugs. The first tier was apparently too hard for most players, though I didn't have a problem pugging it. But the 10 and 25 man sharing a lockout was initially a disaster. It also cut the content in half. While i hated running the same raid twice in WoTLK preferring something aking to ZA or ZG on the side of main raids, it was still something to do.
The initial content was also devoured rather quickly since Blizz probably spent so many resources doing the world over, which most people hate anyway. Blizz couldn't keep up and gave us a rehashed ZA and ZG, where the loot pretty much made the raids not worth running so we were stuck running the same Heroics 2 over and over.
The Firelands patch was good. Molten Front, and the raid in terms of boss design was some of the best I've seen up to that point. But it was a very short raid and people got the itch very quick. The Abysal Maw Raid was cancelled, which was supposed to go along with that tier.
Then Dragon Soul....this is where Blizzard lost their fucking shit! Despite seeing how disastrous LFG is was in the long run, they decide to exacerbate the problem with LFR. Still probably thee worst thing to happen to this game. Back then LFR was completely mandatory, and you were forced to deal with that asinine mess. Also killing Deathwing in LFR as your first kill on him was so underwhelmin, with all the lead up to him you could just auto attack im to death. Where as in Firelands I managed to kill Ragnaros in a pug and it felt great!
At least you could see the effort put into Cata though, with poor decisions and resources going to the wrong place. WoD it's like they stopped caring.