For me personally, I felt like there were three major problems with Cata.
1) The new talent system. I hated having fewer talent points, other people loved it. This is actually the main reason I quit during cata and won't resub now. It's all personal, but I liked getting talents every level.
2) Retribution nerfs. Not as big of an issue, I still mained as Ret, and played a ton of alts while I was still active, but I didn't have nearly as much fun with the completely overhauled ret.
3) Herioc dungeons. Flame me all you want, but I loved Wrath heroics, Cata not so much. I feel that heroics should be more accessible to PuGs while raids should be moreso for the hardcore. Vanilla/TBC raids with Wrath heroics would be my personal ideal.
There were also a lot of things that I felt Cata did right, but for me overall it just wasn't an enjoyable game anymore. Mists made my main gripe even worse, so I haven't come back to play it at all. Although as an addon developer I did get a beta key which I used to try out the monks, but just didn't really wow me like the game used to.
So, all in all, not a bad expac per se, but it definitely wasn't the game I personally wanted.