I second the love for code hot-swapping. That's a beautiful part of the soft-realtime OTP environment that I miss a lot. Lager also helps to mitigate the logging pain a bit.

I constantly missed Haskell's pattern matching whenever I used Erlang's. Same idea, but H-M typing and laziness make me feel less like every pattern match is a bomb that triggers on every refactoring.

Edit: Oh! And how can I forget the joy that is Erlang's binary type DSL! That thing should be everywhere regexes are.