The funny thing is that unlike some other characters with radically different designs in Ultimate Marvel, Cap's outfit is exactly the same, except for the Wing tips. Well in the modern time period. During WWII flashbacks he has a kind of infantry outfit in his later costumes colors on, with his original triangular shield (but I always personally liked that outfit). As for the other guys, they're just getting an update to their current regular Marvel verse designs. Thor's had that extra silver plating ever since he came back after being inactive for awhile, and Iron Man's armor is being updated to look like his current ones.