You can go both ways with the casting of Cap though. You can have an unknown or you can have an actor, the script needs to be great and the film has to be great too. That's where Superman Returns failed in my opinion. The cast was good, it's just the script was pretty damn bad and offered no character development for Superman, Lois, Luthor, and etc.
If they get those basics of Cap right, then it should be smooth sailing. I really don't know who I would pick for Cap though, he's a pretty hard character to cast honestly. In order to cast him he definitely needs to have the "look" if you will of someone who is in control, in perfect shape, and just looks the part.
I just hope we get an official picture that is something like this ^
or