He's responsible for the quests getting goofy and the constant pop-culture references. Metzen still ultimately had the reigns at that time, but it remains a great black stain on his legacy. Kosak's influence is overstated, however. The real issue is that, after the development of Mists of Pandaria, most of the remaining old blood moved on to Titan, including Metzen. That's why the base expansion is actually really solid, and when you read about Shao Hao and the Celestials, it comes across like the developers were addressing the player, rather than the character. It's a team effort, though, and plenty of the quests are completely retarded and Pandaren are a huge joke, but I always got the sense that MoP was our send off from the remains of the original team.
Then they moved to Titan, it died in their arms, and the scrounged up Overwatch so it wasn't totally a wasted effort, but they never really recovered from the ordeal. You saw a lot of people leaving Blizzard after that, including Metzen, who commented on how defeated they felt after Titan fell through. Once Morhaime left, that was pretty much it. Nothing's left.