I briefly mentioned this during the Twisted Nether interview, but I think it deserves another saying: Cataclysm was not a horrible expansion and would have been a pretty awesome game by itself. The problem is Wrath of the Lich King. Or was.
What did LK do? It gave WoW a whole lot of problems. Cataclysm, at worst, built off those, but in terms of truly creating new problems, Cataclysm didn't do a whole lot wrong (beside retaining LK problems).
Problems that LK brought to WoW:
- Taught us that heroics are meant to be trivial zerging lootfests. Cataclysm tried to fix that and make heroics more challenging and presumably more fun. But once people have been trained to expect something, especially if that something is a lot of rewards very fast, they don't like change.
- Brought about extensive use of phasing. Cataclysm extended this. If anything, Cataclysm did it better, using phasing in solo-oriented content such as leveling. LK used this anti-grouping mechanic (fall behind and the other person must wait, cannot help) for lead-ins to group quests and instances. Phasing also encourages, even requires, highly-linear quest designs.
- Random formation of cross-server groups. Your mileage will vary in terms of how much you care about the negative social effects relative to the speed gain. But I do think that BC had higher-quality groups than LK and beyond. We could blame this on a population change, but that would assume data that I don't have, whereas we can see how the devs changed incentives and social mechanics with these features.
- Everything became about raiding. Now certainly vanilla and BC had a lot of raiding going on and a lot of raiding content. But there was not an expectation that everyone would raid or must raid. Both had players who wanted to raid but could not due to this or that problem unrelated to skill, and so changes were almost certainly to their benefit. But I suspect that a lot of players who did not particularly want to raid were driven into it by the structure of the game. Late-game (not end-game) content was thinner, in favor of fast gearing to get people into raids. This causes all sorts of problems, such as players who lack internal motivation to raid and who must instead be driven purely by loot or social pressure, neither of which are long-term formulas for fun.
- Blue sets, what? BC had blue-quality late-game sets. They were not fantastic at all. But some pieces were okay. My paladin used a couple pieces for a while. Other classes had similar experiences. These were not amazing sets, but sets give some sense of legitimacy, they say that this tier is one that is okay to be at. Sets indicate that the devs think you will be here a while and are okay with rewarding that. LK had no instance sets, nor did Cataclysm. It says something: don't stick around and do not, under any circumstances, feel like anything you got here is worth remembering or keeping.
I'm not claiming that sets imply we are meant to stay at a certain level of content, since I doubt we're meant to spend months in VC or WC (both have sets). But the lack of a set does indicate that content is meant to be temporary and fleeting.
- I will do the math so I can properly blame LK for jewelcrafting (since BC is currently the best time ever in WoW, having replaced vanilla despite neither changing), but that's a bit too complex for right now. The number of gems isn't the problem (I checked).
- Time. LK gave WoW a new way of doing thing. Some was good, some was bad. It took time for people to get sick of the bad and quite, which coincidentally put it during Cataclysm. Also, if not for LK, Cataclysm would have been sooner and there would have been fewer horribly burnt-out veteran players leaving.
If WoW had Cataclysm, minus LK, it would be in a much stronger position.