Shiny Venusaur
Sinnoh's Champion
I wouldn't say that I miss this specific mechanic, but one of the few mechanics that are genuinely unique to Generation 1 is that Body Slam cannot paralyze a Normal Type. When this mechanic was discovered years past RBY, it drastically shifted the RBY meta, which was incorrect on simulators for years since no one had caught this very specific hidden move effect.
Bulbapedia - Body Slam (move)
Bulbapedia - Body Slam (move)