You know--I can't see a huge difference either. But we did datamine two models and the files had them called Vol'jin/Vol'jin Wounded, so I figured I'd put both of them in so players could compare (and also confirm Vol'jin would be attacked in some way).
The wounded version of Vol'Jin has a cut/scar on the left side (his right side) of his neck. You can barely see it, or at least identify it as such on the static picture, but you can see it more clearly if you watch it in 3D.