^ This
Sure, one can look back now and say that Cata wasn’t great for one reason or another. Hell, I personally feel that Cata doubled down on some things that still hurt the game in various ways to this day (a topic for another time). However, at the end of the day, I feel calling it the “worst” expansion is a massive stretch.
In a world where WoD exists, I honestly don’t know how someone can compare Cata to that expansion and say that Cata was worse with a straight face. I can easily say that Cata certainly doesn’t rank among my favorites looking back, but much like SL (which I do feel is just barely above WoD) it was functionally a working expansion that did bring in a new zone post launch, some new instances, and ultimately ended with three distinct tiers of raid content.
Ultimately, that’s the low bar:
- Three tiers of raid content
- At least one new instance (or scenario/delve as other expansions have leaned into those instead) at some point post launch
- At least one new zone at some point post launch
WoD did NOT give us three distinct tiers of raid content. It did NOT give us a new instance post launch. While it did give us Tanaan Jungle later, the fact remains that the zone SHOULD have been in the game in an uncorrupted state from launch with 6.2 then transforming it later on. Patch 6.1 will forever be the worst “major” patch in the history of this game due to the sheer lack of anything meaningful, and it is very much a reason why “minor” patches now use the X.X.5 numbering system as it vastly helps temper expectations for what to expect.
In terms of the three worst expansions? I personal think you’ve got WoD at the very bottom for the previously mentioned reasons. Granted, it DID start with a strong hook/premise, but a good story alone can’t save a questionable expansion. Hell, it quickly flushed that away and killed well over HALF the villain pool IN PATCH 6.0! That’s followed by SL as it was functionally a working expansion based on the guidelines I listed, but the overall story was atrocious between the questionable choices and retcons, and it very much repeated much of what Legion/BfA did with the AP and borrowed power systems which were already getting very dull halfway through BfA.
Cata sit just above those two IMO. I wouldn’t call it outright terrible, but once WoD/SL are removed from the list, I cannot find a single expansion that I feel was worse than Cata, so it essentially becomes “the worst” by default if WoD/SL don’t exist 
Well, I said it was another topic for another day, but I suppose we can touch upon it now that it was brought it.
This was ultimately my issue with Cata. People got so used to overgearing stuff at the end of Wrath that they forgot that new expansions were a gear reset and that you had to start using your crowd control abilities again. Instead of adapting however, people complained that it was “too hard!”
The biggest mistake Blizzard ever made was dumbing things down post launch. It wasn’t that things were hard: it was the fact that folks just wanted steamroll over things with little to no thought process. It also ultimately taught the players as a whole that if they complain loudly for long enough, Blizzard will ultimately cave in and give them whatever they want.
That is the mistake we’re still paying for to this day. People started chain pulling in Wrath because the gear supported that mindless “GO GO GO!” nonsense. Blizzard gave in at the start of Cata and essentially made that the gameplay model moving forward. Remember that the next time someone pulls way more than they can handle (or has impatient DPS try to “help” by pulling too many extra mobs), or a player refuses to interrupt spells, use Sap/Poly in a situation where it might legitimately be a useful tactic, or someone stands in fire (yes, it’s always been a thing to a degree, but it got WAY worse post Cata!)
