In peace, there’s nothing so becomes a man,
As modest stillness and humility:
But when the blast of war blows in our ears,
Then imitate the action of the tiger;
Stiffen the sinews, summon up the blood.
The title of Warchief is used to denote the leader of the Orcs in WOW. It's a term with a very clear implication: that the primary role of a leader of the Orcish people is to lead them into battle. Equally obviously, it's a role that Thrall has gone far beyond. He's a leader of the wider Horde, a spiritual leader through his shamanic links to the Earthen Ring and a subtle politician who sees the futility of the endless conflict between the Alliance and Horde.
From our external perspective it's easy to see Thrall as the perfect leader and very hard to see how anyone could possibly think Garrosh was fit to lead in the same way. I've made quite a few posts on the topic myself!