Let's be honest runs are runs. Yes runs against tough opposition are more valuable and stick longer in the memory, but all greats do buffer their averages against poorer opposition. Recent examples that spring to mind being Amla against the Windies at home, and Sangakarra throughout his career against Bangladesh (triple centuries etc.). That's not to discredit them as players, as the runs still take scoring otherwise everyone would be able to do it.
Root is a fine player and has a good head on his shoulders. He will be world class if he can sustain anywhere near this level of maturity for the next few years, and translate it into one day form. Just because he couldn't open against NZ or Aus doesn't make him gutless. De Villiers could never be an opener, but that doesn't make him any worse of a player. They are both stroke makers and accumulate runs very quickly (not to say they're on the same level yet).