Red Skull makes quite a few allusions to Hugo Weaving's role as Agent Smith in The Matrix.
His real name, Johann Schmidt, is a coincidence because that's the character's name in the comics, but it sort of counts; "Schmidt" is the German form of "Smith".
His introductory scene has him approaching in a car, and then walking into the church hideaway with his feet being focused on first, and Erskine's explaining about Red Skull has the first flashback depicting him with several copies of himself, similar to a certain Agent in The Matrix. The shot of a dozen Schmidts is a particularly strong example as it makes absolutely no sense except as an Actor Allusion.
His fight with Cap in a plummeting plane, which makes a suspended-gravity environment, is reminiscent of a scene in The Matrix.
At a meta level, things like Red Skull's fighting style and even how he levels his gun ("Unfortunately I am on a tight schedule...") have apparently stayed with Hugo Weaving after the Training from Hell he went through to play Agent Smith.
Australian Hugo Weaving, American Stanley Tucci, and Englishmen Toby Jones and Richard Armitage play Germans.
Englishman Dominic Cooper as Howard Stark, American playboy and captain of industry.
Englishman David Bradley as the Norwegian keeper of the Cosmic Cube.
Romanian-born Sebastian Stan as American Bucky Barnes.
Was actually deliberately averted in the case of Captain America himself (Sebastian Stan originally auditioned for Steve). The crew felt that it was necessary for the Cap to be played by a genuine American.
Thoroughly averted in the case of Neal McDonough as Dum-Dum Dugan, who shares the same nationality and home city of Boston, Mass, as well as Irish lineage, as well as Bruno Ricci (who is actually French) as Jacques Dernier.
English Natalie Dormer as the American soldier who kisses the Captain.
Korean American Kenneth Choi as Japanese American Jim Morita.
Fan Nickname: Pre-serum Steve is often referred to as 萌やし (moyashi) or "beansprout" in East Asian fandom due to his small size.
I Knew It: As confirmed by the sequel, Bucky survives and becomes the Winter Soldier.
Money, Dear Boy: Hugo Weaving has gone on record stating that he is indifferent to the film and has very little interest in reprising his role as the Red Skull, due to the difficulty of the makeup process involved.
The Other Darrin: Howard Stark was portrayed by Gerard Sanders in still photos, John Slattery in archive footage and Dominic Cooper in life. Justified to an extent as all three depictions are from different time periods — the still photos are from around Tony Stark's pre-teens and beyond, the archive footage from his childhood, and in real life it's the '40s long before he's even born, so Howard ages significantly between each depiction.
Sebastian Stan originally auditioned for the role of Captain America, but was given the role of Bucky Barnes.
Chris Evans turned down the offer to play Captain America several times; not out of dislike for the role, but because he feared what the effects of the sudden increase of fame would be on his private life. Then Robert Downey, Jr. convinced him to take the part and thus gain the freedom to sign on any other role he'd want to after, and the rest is history.
Up until a very late stage in pre-production, HYDRA would have been a blatantly Nazi paramilitary organization, with swastikas on their uniform. The implication is still present in the final version, however. In addition, deleted scenes also had HYDRA explicitly attacking Nazis in addition to Allied powers.
Early drafts of the script featured Baron Zemo and Baron Strucker working alongside the Red Skull. They ended up being cut because Christopher Marcus and Stephen McFeely felt the characters would've been "wasted" in mere supporting roles.