This post is recommended AFTER you have watched Captain America: The Winter Soldier, unless you don't mind spoilers. So stop reading as you have been warned. To read the below, just highlight the text below!
So let's start with the picture above. If you're not familiar with that character, he's Baron Von Strucker who appears in the mid-credit scenes at the end of Captain America: The Winter Soldier. He is not dressed like that, but he is a leader of Hydra, a former Nazi officer that is confirmed to be in Avengers 2. He is believed to have survived through the ages with the same experiments that have kept Bucky around.
Not a really big secret but The Winter Soldier is Captain America's long presumed dead friend "Bucky" Barnes. In the comic books, this story line was written by Ed Brubaker in 2005. Bucky's memories was finally being restored by Captain America with the Cosmic Cube. Whether this will happen in the movies remains to be seen.
Agent 13, Captain America's neighbor who was sent by Nick Fury to guard him, is named as Sharon towards the end of the movie by Natasha. If you remember in her first scene when she's heading down for her laundry, she was talking to her aunt. Her aunt is Peggy Carter, making her Sharon Carter, a key character in the world of Captain America comic books. She eventually becomes a love interest of Captain in the comic books, so it'll be interesting to see if they do so as well in the future.
As also part of the mid-credit scene mentioned before with Von Strucker, we see two young adults (or older teens if you prefer) imprisoned using their powers. The speedster is Quicksilver and the one levitating objects is his sister Scarlet Witch. Of course in the comic books they are children of Magneto. But with Fox Studios having the rights to X-Men and mutants, the duo is explained by Von Strucker as survivors of an explosion. An explosion presumably an experiment to make super humans? Quicksilver and Scarlet Witch in the comic books do join the Avengers at different periods of the team history. It'll be interesting to see what role they have in Avengers 2 of which they're slated to appear. It'll also be interesting to see if Marvel will explore Quicksilver's rather odd over protecting of Scarlet Witch, which you might even call an unhealthy infatuation.
Regarding Sam Wilson, the Falcon, his origin in the comic books is vastly different from the movie. He has no military training in the comic books, but rather meets Steve Rogers on an island and receives training from Steve to be a fighter against some Nazi supporters that are backed by the Red Skull. In the comic books, Sam also has a Falcon named Red Wing with whom he develops a type of psychic bond with fighting crime together. Created by Stan Lee, he was the first African American super hero, with the Black Panther being the first African super hero.
Towards the end of Captain America: The Winter Soldier, we see S.H.I.E.L.D. agent who turned out to be a HYDRA double agent Brock Rumlow taken away in a stretcher with all sorts of his damage to his body. Expect him to return in a future movie as the trigger happy villain Crossbones. In the comic books, his training is from the super villain Taskmaster, and becomes a mercenary that eventually joins the Red Skull. In the comic books he also eventually has a relationship with the equally sadistic daughter of the Red Skull, Sin. I wonder if she'll make an appearance in an upcoming movie if it includes Crossbones.
Finally, the after credit scenes shows the Winter Soldier visiting the Smithsonian which has the Captain America display shown earlier in the movie. I actually missed this as I didn't stick around after the mid-credit scene. I am told he's looking over the display of him as Bucky along with the information about Captain America and himself.