> Immutability is cool, but we only needed to not be able to pass mutable references on the message bus, not be fully immutable within an Erlang process. I believe Elixir fixes this one.

In fact Elixir does nothing for this. It is true that in Erlang, you cannot rebind a variable and must use a different variable name for every assignment. In Elixir you can rebind the same variable, but the data is just as immutable as in Erlang, and the rebind is only visible in the current scope, which trips up new programmers not used to it.