Angst? What Angst? Such a powerful aversion, that it's worth mentioning. As soon as Mortimer sees a corpse in his aunts' house, he loses his composure. He can't talk, can't whistle, can't think, and virtually forgets about his bride-in-waiting. Though he eventually musters enough control to try and expiate his aunts of twelve murders, he is never quite the same throughout the entire feature. His sudden absent-mindedness, slurred speech, physical jitters, and paranoid bewilderment are a good reminder that seeing a dead body could easily alter one's temperament.
Harsher in Hindsight: Mortimer's comment about how one of his ancestors "scalped the Indians" as proof of how crazy he was...in fact, historically whites DID scalp Indians throughout the various conflicts between them. However, since most white Americans didn't know these facts when the movie takes place, it doesn't break the Suspension of Disbelief for the audience when Mortimer tells this story to Elaine.
Nightmare Fuel: Jonathan. The fact that he's in a film where everyone else is just charmingly kooky makes for quite a contrast.
Special Effect Failure: In the outside scenes, the obvious backdrop of the Brooklyn Bridge and Manhattan is jarring to modern audiences.
Values Dissonance: The treatment of mental illness hasn't exactly aged well. Not only is mental illness mocked, but there's a strong implication that the mentally ill should be segregated from the rest of society. No one finds it at all surprising that the police want to involuntarily commit Teddy for a series of noise violations (sadly, was Truth In Televison at the time).
Values Resonance: ...or has it? The obviously mentally ill character, Teddy, is harmless, and loved by everyone, to the point that even the villain's henchmen puts his foot down when the idea of harming Teddy comes up; the homicidally insane characters meanwhile are the seemingly sane, well-liked aunts. The idea of sending Teddy to Happy Dale isn't much different than what elderly parents often plan for their mentally disabled children, and the asylum is implied to be a pleasant place, not the den of deadly lunatics that many other films in that era portrayed asylums as.
And mentally ill serial killers should probably be segregated from their potential victims.