You don't often run into discussions about what isn't in the Haunted Mansion, and when you do, it's usually to note that there isn't any grue, or there aren't any big scares. Otherwise, it's what's there that gets the ink or the bandwidth.
There are, however, two omissions that create openings for some interesting observations (interesting, at least, to Long-Forgottenistas, or whatever it is we call ourselves).
Children Not Admitted
There are no ghosts of children anywhere among the 999. Nada. Zip. Zilch. The closest we come is a mute admission that children exist within the imaginative world of the Haunted Mansion. Amidst the junk in the attic have been prams and hobby-horses at one point or another, perhaps an acknowledgment that some of the Mansion residents were once children. Not exactly news that will make you spill your coffee.