Abbott and Costello Meet Frankenstein is considered to have brought an end to the reign of the Universal Monsters. Although some other memorable monsters followed in the film's wake (ie, the Creatures from the Black Lagoon), the theory is that pairing the monsters with Abbott & Costello either made it impossible for audiences to take them seriously again (the monsters, that is, not Bud & Lou), or it was simply a blatant cash-in product intended to creak one more film out of Universal's horror stars and effects.
What people sometimes forget is - man, this film is scary when you're young! Especially when you have some familiarity with the monsters involved. I spoke yesterday about the impact the Wolf Man had on me because it allowed its own hero to die. If heroes can die, then were Budd & Lou truly in danger from the monsters? The scene where Lou is comically unaware the Wolf Man is stalking him had me on the edge of my seat as a child.
Of course, I also found Abbott & Costello very funny as a child and would eventually see nearly all of their films. But what I remember most about that first late night viewing are the scary scenes; even the animation-derived bat transformations Dracula underwent seemed spooky to me.
Perhaps the strongest effect came from the film's heroine being hypnotized by Dracula. At one moment, Lou looks into her eyes and sees a bat reflected in them. I recall the next morning staring very concernedly at my sister's eyes for similar evidence. She laughed and accidentally spat in my face.
At least I knew she wasn't hypnotized.