Well if Vol'jin loses at least he loses to Thrall, and not Sylvanas, Varian, Mekkatorque, Vellen, or Lorthemar Theron, Vol'jin may be the best but in a format of which they are fighting, I can see Thrall, Tyrannde, or Muridan beating him (I'm not saying losing to Carine is dishonorable, or he can really kill Vol'jin he probably wouldn't survive long against him though.). But Vol'jin has long ranged dark magic, and DoTs (Like a shadow priest I think), and then he has a bow, with poisonous arrows (A hunter without a pet), and he can stealth in the shadows, and strike from behind quite easily and is hard to spot and is very agile (Not really like a rogue, though he has aspects of it), then comes his Dagger-y, sword weapon thing, he can use that as back up, or if the enemy is in the dead zone of his range, if he can't shoot he can stab. I may have made a bad explanation (Sorry) but he is a shadow priest, and a hunter, with a few rouge like aspects, versus Thrall a warrior like Shaman, and if you want to measure pure power thrall is the strongest shamans in the world, and Vol'jin is one of the world's greatest rangers (He is out classed by sylvanas from what I hear), and is an amazing user of the dark arts, he was able to turn Rexxar and Thrall (I think...) into invisible Wyrnves, so that implies he will be able to shape shift, and he is said to be able to move from one place to another at lightning speed (He might be that fast, or just using magic), and then his is known as a brilliant tactician.
(The post really carried out, I can tell its a bit bad, and repetitive but I don't want to edit it because I don't want to rewrite it, sorry).