Vol'jin has literally done nothing making him worthy of this title.
He made a rebellion, woo, well, he wasn't able to rescue his people. Even when he returned to sen'jin, he just stood by well Thrall and Chen faught off the kor'kohn.
You know who it was fighting to take back undercity in wrath days well vol'jin sat outside and did nothing? Thrall and Sylvanas.
For years, Vol'jins done nothing but stand around in grommash hold. He's not done anything worthy to validate this. And honestly, all this feels like it was done for shock value, almost as if the devs were saying 'here, we're show you we can choose someone else beside an orc', and thats it.
Maybe if the developers had spent more time developing him over the years, instead of just paying a writer to write a novel for him, and claim that as development (which from all accounts, it had nothing in it as far as development went for vol'jin) then maybe I might have felt far less diappointed in this.
Seriously, Rexxar has done more for the horde then vol'jin.