Scheme's expression comments are really quite handy, but only possible to implement in languages where everything is an expression.

For those not familiar, you can have the reader just discard an expression, so like (list 1 #;2 3) becomes (list 1 3), but you can handily comment out a giant function call or if expression or something while you work on it.

Scheme's nestable multiline comments are also nice. I'm not sure which language was first, but I think more languages are getting them now (Kotlin has them IIRC).