10. That He Was Repeatedly Killed Off
In the early years, the Clown Prince of Crime was very much the Kenny to Gotham City's South Park. In fact he was supposed to die at the end of his first appearance in Batman #1, following a story where he demands millions of dollars in ransom lest he poison a succession of high-powered businessmen, leaving them with his trademark rictus grin.
The editors at DC knew they were onto something with this new villain, however, and forbid the writer-artist team of Bill Finger and Bob Kane from shuffling him off his mortal coil before they'd had chance to properly exploit him. Still, the idea of comic book death - that any major character who was bumped off was inevitably going to return through some tenuous means - began in earnest as the series entered the forties and The Joker was struck down at least three times, from being stabbed in the chest by Batman himself to accidentally getting flattened by a train, only to reappear in the next issue with a whole new scheme with which to menace the dynamic duo. Usually with no explanation.
Not that his untimely demises ended in the Golden Age, necessarily.