He seems to polarize opinion on here - either (i) too rash, prone to lapse of concentration and ultimately not good enough, or (ii) is a tough bastard who is a bit of an all rounder and settling well into the team.
I'd probably go with the latter, he has that fire in his belly, temperament to battle away against the top strikers. His got a lot of the physical attributes pace, strength, decent height and aerial ability to deal with both the quick forwards and strong ones. The worry is that he is prone to the odd lapse of concentration but Rio Ferdinand at a similar age was having problems with that. Rojo is still only 24 and it can be drummed out of him with plenty more games and experience.
Most of the current players in the squad lack of bit of tenacity and grit and Rojo brings some of that.