Of note is that Blizzard felt spread too thin with the ambitious revamping of the entire old world. That lead to less-than-ideal elements in the current endgame experience, which in turn is a key component of many player's dislike of Cataclysm.
Say what you will about people's complaints, but it's a good thing that Blizzard is able to look at itself with such a critical eye and fine tune its future processes to make things even better.
Some of the key points:
- The primary goal of Cataclysm was to remake the 1-to-60 experience, keep it relevant to new players.
- Blizzard is happy with the old world revamp.
- Blizzard felt spread too thin, which resulted from the total revamp of the old world.
- The 80-to-85 zones don't feel as connected to each other as Blizzard would have liked.
- "Phasing is like a story sledgehammer."
- They want future phasing to not split players up but to provide visual changes only you can see (like in the 4.2 dailies).
- "Cataclysm was in many ways Thrall's story."
- Future legendary quest lines will be a lot like the Fangs of the Father quests that rogues are experiencing right now.
- Blizzard is refocusing on core gameplay mechanics.
You can also take a look at our own post-mortem of Cataclysm's zones we did in late 2011.