Here are Coburn's biggest weaknesses:
Phaneuf, Seabrook, Weber, Suter and Carle.
Coburn WAS rated HIGHER than Dion Phaneuf going into the 2003 draft, so the Thrashers didn't go way off the board. If anything, Coburn was a better player at the time.
I haven't seen much of Coburn, but to me he projects as a second-pairing guy, a jack-of-all-trades but master of none type of guy. Solid, but doesn't produce a ton of offensive output or deliver huge hits routinely. Just a steady guy. Nothing wrong with that. How many young defensemen, even Top 10 picks, even turn out to be consistently solid NHL'ers? Not as many as people realize.
Just because he's not Seabrook (and that selection was widely criticized if I remember correctly...those people got owned), Weber (I really like this kid...he's my kind of defenseman), Suter (who actually reminds me of Hamhuis, kind of) or Carle (this kid has a better overall game than people realize, not just a scorer) doesn't mean he's a bad player. He just has years to go yet. If he was picked that high in the '04 draft, no one would be complaining.
Coburn's biggest burden in the eyes of many fans (although it shouldn't be) is "Celine" Dion Phaneuf. I don't know if anyone could have predicted the simply dizzying rate of development by Dion from 2003-2005. I don't know if I've seen a CHL defenseman improve that much that quickly during the time I've been a hockey fan, and if there have been guys who fit that criteria, they do not come to mind right now. Plain and simple, 19 and 20 year-old defensemen do NOT make and EXCEL in the NHL without AHL time. It just doesn't happen. Phaneuf is an absolute marvel. He didn't just make the team. He made the team and played Top-20 D-man hockey FOR THE ENTIRE LEAGUE. Amazing. WHAT A MONSTER.