One topic that's frequently revisited throughout the post-mortem is the talent system revamp. In a nutshell, Ghostcrawler says that the team (and the players) were largely pleased with the ability to choose a spec and gain powerful, useful skills right at level 10. What didn't work out as planned? "Everything else!" says Ghostcrawler. The team realized that "the talent tree model where you pick up tiny performance increases here and there (and where there's, mathematically, nearly always a 'right' answer and a 'wrong' answer) is not a great model." He goes on to add that Mists of Pandaria's talent system "should fix this problem once and for all." Well, third time's the charm, right guys?
At any rate, there's more on the discussion table than just talents, so head over to the full article and see what lessons Cataclysm has taught the devs.