Garrosh never filled any kind of pragmatic leader role. He was a fool from his introduction in BC and all the way up to his death, and never managed to display any redeeming qualities in-between.
Despite this, he was a good villain. Love to hate him and all that. Sylvanas has none of that. Her fear of death explains how she has gotten this way, sure, but I don't really give a shit about her. She's a rabid dog that needs to be put down.