I actually liked the first tier of Cata quite a bit. I liked the return to more difficult heroic dungeons and liked being able to explore two different raids - and I liked the boss fights in those raids. Some fun and engaging encounters. I also enjoyed leveling up to 85, the high levels zones were well done, I thought, if a bit too linear for my liking (Deepholm is one of my favourite zones). Things went steadily down hill from there, unfortunately. Redoing ZG and ZA into 5 mans was not a good move in my opinion - maybe that's connected to my nostalgia for those places when I raided them as current content, so biased opinion is probably biased. Firelands did not grab me. I know a lot of people liked it but the environment gave me molten core flashbacks.
I think Dragon Soul is the worse end of expansion raid - and second only to ToC in the race for 'worst raid ever', for me. I was very disappointed by the re-used environments, the fact that we just portalled from random placed to random place instead of exploring and conquering an enemy stronghold, it just felt terribly uninspired. I enjoyed the Spine fight but then Madness was just.... what? We're fighting Deathwing's... tentacles? And then we had that for months on end. *Shudder*
Edit: Oh and transmog. I can't completely hate the expansion that brought us transmog