Obviously he shouldn't be playing center, but even in right he's probably only average defensively right now (maybe a small tick above), and that's going to decline as he ages. On the hitting side, he's not a big power guy, so as his bat slows the power he has will likely disappear which will decimate his OPS. His eye will still make him a decent player, but not at the money he'll command. That's just my opinion. He could re-invent himself or buck those expections, you never know, you just can't count on it happening.
I believe according to the 2013 pitch framing study, McCann has saved the most runs of any catcher between 2008 and 2013. Further, he's been rated as a positive defender in every year since 2008. He's 6th in WAR for catchers this year in spite of missing part of the year and only having 2/3 of the games of other guys on the list. He's 4th in WAR and 3rd I believe in defense among all catchers since 2010 behind only Wieters and Yadier.