Ah, the great divider. The end of an era. Cataclysm has a pretty miserable reputation, which is a shame, because there's a lot of good stuff in Cata. The new 80-85 zones and the new starting areas for the Worgen and Goblins are all great. The revamped 1-60 leveling was actually fantastic, and it's a massive shame that everyone just sat in their capital city and spammed LFD because it was faster... classic example of players ruining a game for themselves. LFD also more or less ruined the new heroic dungeons. They were hard in a way that was very satisfying and well designed... if you had an actual guild group. Trying to complete them with LFD randoms was true horror. The new raids were pretty great for the first two tiers as well.

Unfortunately Cataclysm was just too ambitious for its own good. It shouldn't be any surprise that there ended up being a lot of cut content and somewhat of a rushed, undercooked end to the expansion. The team had little time to work on the next expansion, so the final raid (which was considered lackluster to begin with) was current content for an extremely long period of time. That last third of the expansion really left a bad taste in the mouths of many, many players. Releasing at the same time as that final raid was the most divisive feature ever added to WoW: LFR. The very concept of LFR was sacrilege to many players, including myself. I've definitely softened that stance over time, but even now LFR's presence has a very negative effect on my motivation to play the game. Even the game's story was starting to drive players away, as players perceived a frustrating amount of favoritism being shown toward Thrall, who is typically seen as a self-insert character for Chris Metzen, WoW's creative lead at the time.

To many players, the revamped geography of the old world serves as an eternal reminder that the World of Warcraft that they know has been destroyed. Ashenvale has a giant volcano in the middle of it that never stops erupting, Westfall is ravaged by a perpetual tornado, the Barrens have been quite literally torn asunder, etc. Many players no longer feel at home in Azeroth because their version of this world died in 2010. So much of Cataclysm is great... it's such a shame that so many only remember it as the death of something that they once loved.

Despite the negativity this expansion gets, this was where dungeons took a step in the right direction. Mechanics were more prominent, Crowd Control was actually required, each class was more well-rounded for encounters.

As for everything else this expansion changed... Welp lets not pretend like it was all good before this. Each expansion (+Vanilla) before it tore at what WoW was and the different reasons people liked it.

Before Cata, Vanilla had lackluster lore around raid bosses.
Before Cata, TBC changed raiding for better or worse. (Depends who you ask.)
Before Cata, Wrath really turned questing into what felt like a chore instead of a necessary process.