Blizzard is a company with a lot of career advancement opportunity if starting at the bottom but he was at a point where no amount of years at the company would let him go any higher unless some of their "lifers" died or left the company which were both unlikely to happen anytime soon. He could stay at Blizzard and do the same job or he could take a senior development position at someone like Riot Games.
I find it depressing how many people truly think it was Ghostcrawler who ruined the game. He gets blame for
-PvP: Hollinka's team and before him, Chilton had a specific team who did the bulk of the legwork here even as the lead game designer
-Dungeons and raids: This used to be J. Allen Brack's team up up until the reins were handed to Ion Hazzikostas and they settled on a rigid team.
-Lore/Quest: This was a joint effort between Metzen to write it, Dave Kosak to implement it, and Cory Stockton to polish it.
-"Fluff" content: Things like pet battles and the new garrisons are a Cory Stockton area
-World design: This is Alex Afrasiabi's area. The no flying debacle is on him as well.
The main things Ghostcrawler did were dungeon finder/LFR, transmogrification, badge/point loot, itemization, and class balance. Of course classes became more homogenized over time but it was largely 10H raiding that forced them to do so to ensure that all classes could be brought. Class balance in particular is better than it's ever been.
Ghostcrawler was simply another name on the team under Chilton with his own specific area no different than the others I mentioned above. But because Ghostcrawler was the one to come to the forums and was willing to discuss issues and explain changes made by the WoW team as a whole, he got a lot of hate directed at him and a lot of people overestimating his power on the team. If people are unhappy, they should be directing their anger at Tom Chilton as the game's director and lead game designer; the manager of the entire WoW team. Or even a step farther, Rob Pardo who's the VP of game design over all Blizzard games. But it's easier to blame Ghostcrawler as he's the most easily accessible member of the team through his active Twitter feed.
Blaming people for unhappiness is fine but people at least need to blame the right people.