Good stuff to read (or watch/listen) here and here on +Justin Welby to be ++Justin Welby, Rockstar and Leader of Canterbury - ++Rowan has been the Rockstar and Theologian of Canterbury. I suggest what ++Justin will bring to the role is "leadership."
I am excited about this appointment. It demonstrates wisdom on the part of the CNC as well as a determination to find the right person for the role, no matter what their perceived episcopal experience is or isn't. It happens, as a matter of my own personal biography, that I once met Justin while he was an ordinand at Cranmer Hall. Even then it was clear that Justin was 'bishop material'.
I want to be realistic about this appointment and not invest unreasonable hopes and expectations on +Justin, especially not in relation to the Anglican Communion.
AND courtesy of a commenter here, thank you, this is worth a look.