Johan Schmidt's 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.
Red Skull (Hugo Weaving) while fighting Captain America in a suspended gravity environment (the plummeting plane) reminds somewhat of a similar 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.
The Other Darrin: Howard Stark was portrayed by James Caan 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.
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.