Dangerous Dave in the Haunted Mansion

"Old lady"

The knife-throwing creature which is first encountered on upper floors of the first level was affectionately called "old lady" by many fans, though it is actually a male hunchback, similar to classical Igor from Frankenstein movies.

Shotgun

This is the first game the original id Software crew wrote that has a shotgun in it.

Violence

Adrian Carmack originally made the art much gorier than it is in the final version. Softdisk wanted the gore out, so he had to tone it down.

Information also contributed by Frenkel