Originally Posted by Juicy J
Anything other than a Cap suit won't suffice. Bottom line.
It makes a tremendous amount of sense to Cap to be dressed in his comic-book styled suit when leading others into battle. He is meant to be a symbol of inspiration, and was to the Howling Commandos in TFA and to the members of SHIELD (Coulson in particular) in the Avengers.
However, it doesn't make a bit of sense for Cap to don his highly stylized battle suit when on a secret-agent mission, and could very likely make him look silly. As in TFA, we'll likely see Cap in a variety of outfits - Civilian garb as in the concept art, a super-soldier type SHIELD outfit, and the full red, white and blue costume for the final battle.