For most of what I think of as the first three ages of comics (gold, atomic and silver), anniversary issues meant nothing. It was assumed that readers were children, with rapid turnover and short attention spans. You'll notice that the smallest thing on the cover is the issue number.
Surely if #300 mattered to anyone, they would have introduced a better villain than the Polka Dot Man. It wouldn't have taken much: The Matt Hagen Clayface had just debuted in #298, and he proved popular enough for a return appearance in #304. Surely the schedule could have been shuffled enough to swap stories.
Ah, but that is the genius of hindsight. I'm sure at the time, the Bizarre Polka Dot Man looked far more promising. You never know. Detective #400 introduced the Man-Bat, who struck me as a one-note character, but somehow he's still around.
Detective Comics #300 marked 25 years of continuous publication. It wasn't quite 25 years of Batman: That had to wait until Detective #327, for reasons you'll know if you think about it. That issue was a big departure from what came before, since it introduced the Infantino "new look" Batman, but still there wasn't a single mention anywhere in either comic of Detective's, or Batman's, silver anniversary.