Alternative Character Interpretation: In the 1962 film, Janet Leigh's character of Rosie (the Love Interest) meets Marco on the train and they have a singularly bizarre conversation. Later she bails him out of jail and through a shorter but just as weird conversation we learn that she's left her fiancee for him despite only meeting him once. Then she has no impact on the rest of the movie at all other than to fill out some dresses very nicely. The very odd nature of Rosie and Marco's meeting and the general uselessness of her character have led some (including Roger Ebert; see his review here) to theorize that she's a Deep Cover Agent sent by the Chinese to keep an eye on Marco. Others guess that she's actually an American agent investigating the conspiracy, as Jonathan Demme did with the character in the 2004 remake.
Word of God (John Frankenheimer) said on the DVD commentary that the scene in question came straight from the book and he had no idea what that bizarre conversation meant.
Narm: The original film has the flashbacks to Raymond and Jocie's prior relationship. The subplot itself is tragic and heartwarming but the montage of Raymond, Jocie, and the Senator laughter so uproariously so throughly Tastes Like Diabetes.
Raymond's outfit at the costume party (a cowboy) makes him look like a child. To a lesser extent, Johnny looks like an idiot in his Lincoln costume.
Protagonist Title Fallacy: In the novel and first film, the Manchurian Candidate is Senator John Iselin, a villain who is Mrs. Iselin's husband, confidante, and pawn. The 2004 film puts its own twist on this, referring to a corporation called Manchurian Global.
Uncanny Valley: The animated video used to brainwash the subjects in the 2004 film is just realistic enough to raise the hackles.