The $200 offered by the bank every time one crosses Go. They were already offered in the original The Landlord's Game by Elizabeth Magie and were there to demonstrate the Georgist idea of the citizen's dividend, a form of basic income.
Possibly also the "Go back 3 spaces" Chance card, being the only card to use a generic board game instruction.
Development Hell: The live-action film adaptation has been on and off in production since the late 00s.
Missing Episode: No 13th episode was taped. Instead, ABC paired the last episode of Super Jeopardy! with the debut of America's Funniest People (which had found ratings success with its aired pilot on May 1).
Old Shame: The 1989 pilot featured Patty Maloney playing the part of Uncle Pennybags and basically being a living token who moved per the dice rolls. The series replaced her with lights on the board.
What Could Have Been: Marc Summers and Peter Tomarken are two well-respected emcees, and they were in 1990. The success of Double Dare meant that Marc was busy elsewhere, though, and Peter's clash with producers over Patty's role being treated as a thing rather than a person meant that he was gone as well.