Anything concurrent and distributed you want supported by your runtime a la Erlang.

The reason is: you want certain guarantees. For example, in Erlang you can set up a monitor on a process, and when that process exits for any reason, you get a notification, {EXIT, Reason}. Even if that reason was a catastrophic failure of some sort.

If this is implemented as a library, then the runtime can kill your code at any time, and you'll never know what happened.