Mat Damon is an incredible actor who has done amazing action sequences in the Bourne movies.
Captain America is a very action orientated man. So Matt Damon so far has the character down. Captain America is a male with blonde hair. Up to that point Mat is still doing well. The further you get to the character he can not do it.
Captain America is a tall man. Steve Rogers is a calm looking gentleman but can appear scary. Matt Damon does NOT have those qualities.
I mean yes, character-wise Mat Damon could perhaps pull off any and all acting that would need to be done for Cap. But physically no.
I recently saw "Invictus" and it showed Mat Damon shirtless for one of the scenes in the locker room. Yes he is buff but he could not pull off Steve Rogers. He is getting too old to do that.
I know this article is kind of short but it gets right to the point. Matt Damon is not qualified to play Cap.
(Sorry all you fan boys out there that disagree with me, this is my opinion)