We know the road map was Holly's idea and she seems to put WoW on track, seems she also up the morale of the dev team by allowing them to work on different things like plunder mode, and smaller features, so I think she is one of the best they had in a long time! John Hight likely agree!
Before people start panicking (like some always tend to, when a blizzard employee leaves), remember that people are allowed to retired, or just change jobs, or do other things. This is normal. Blizzard will hire new, qualified people and the game will continue to grow and flourish.
Man, he was a real one during the dark ages of Blizzard...
Best of luck on the next journey Sir