Feelt the same, I hade so much hope for him(And love), and I really liked it, he was a guy that wanted to protect the Horde, and hade problems to cowork with others with a diffrent mindset, there could been a very interesting story to tell there. But it just ended up him becoming a villain. Which I think is sad. As you said, like they builded up for him becoming the villain, or fail in this case.
As for to HotS Garrosh, I am looking forward to see him! But I must say that I am very surprised, I didn't expect they would do Garrosh over Grom.