Spiders, obviously.
Daleks, and they're still scary.
Rabies (Survivors and also living by the sea, I think there were posters, etc)
Nuclear War (I can remember the grave faces of my parents during the Cuban Missile crisis, then later there was more terrifying info,Threads, etc)
Cancer, particularly leukaemia (from reading old Readers Digests, followed up by DM's alarming explanation)
The Hooden Horse, a sinister Obby Oss type figure that was a traditional thing around the south coast with a scarily snapping jaw.
The bath plug hole, obviously. (Is it me , or are they less noisy nowadays? Ours positively groaned)
Getting stuck in the outside loo. It was brick built and windowless with cut outs at the top of the door. I used to imagine they would have to pass sandwiches to me through these.
There are more, but we'd be here all day…