In the movie, Thor allowed the rest of the Avengers to try and lift Mjolnir, confident that they aren't worthy. Most of them failed, ofc. The Cap, however, managed to budge the hammer ever so slightly, and was the only time Thor's confidence faltered for an instant, showing on his face. Thor obviously noticed what escaped everyone else, even the Captain himself.
Whosoever holds this hammer, if he be worthy, shall possess the power of Thor.
With that in mind, how worthy is Captain America? How/why is he more worthy than the rest but still less than Thor himself?