3. Captain America Doesnt Show Up Until Issue #4
Characters like Cap, Iron Man, and Thor are so closely associated with the Avengers, its easy to forget that Steve Rogers wasnt even a founding member. His body isnt discovered until Prince Namor, the Sub-Mariner, following a defeat at the hands of the Avengers, throws a temper tantrum in the Arctic near some natives who inexplicably worship a rectangular block of ice. Namor smashes some glaciers to pieces, knocking the ice block into the water where it melts. The Avengers, tracking Namor in a sub, discover the body and bring it aboard.
Cap wakes up and is understandably disoriented, but yet somehow aware that hes been frozen in ice for years. He fights the Avengers to prove he is who he says he is and because this is a Marvel comic. Soon, everythings cleared up and they return to New York. An alien menace ends up turning the Avengers to stone, but a sleeping Cap is unharmed. He leaps into the streets and several bystanders react emotionally to seeing the hero again. Remember, this was only about twenty years after World War II - these people remember Captain America from their youths.
This also raises the question as to why Steve didnt attempt to find any of his old army buddies - some of them surely must be alive. Instead, he goes to a hotel and gets a room (presumably paying for it by saying Im Captain America, bitch.) and flips out over television.
At the end of the adventure, the restored Avengers offer Cap membership.