I don't think Coburn will ever be the physical presence that Phaneuf or Suter is likely to bring to the game. That being said, its not like Coburn is soft. He can take the body, and certainly in this early season and pre-season action, has shown that he intends to be much more physical, nasty and play with an edge more than previous seasons.
The strengths of his game are his skating and his puck movement. For a player who's 6'5", he does skate quite well. He's not afraid to join the rush, and looks to have improved his shot a lot over the summer. His release is quicker, his shots better placed, and just seems to shoot with more authority this season.
I don't think Coburn will ever be a flashy Dman, nor do I think he'll fit into any sort of mold... he's not just stay at home, nor just offensive. He is likely to be a top 4 Dman, who does a bunch of things quietly and quite well.
I think one of the things that will be interesting to watch this season (assuming he stays in the WHL) will be if he can be a better team leader than he was last season. He wore the "C" last season, but seemed at times to be bearing that letter as if it was the weight of the world. He again has the "C" this year. Hopefully a year of maturity will help him, and he'll not look quite as overwhelmed by the captaincy.