I love the idea of being able to visualize the state of everything while it's running, for me, that's the killer feature I'd love to see in other languages.
Erlang's VM comes to mind. It has great APIs for inspecting a running system and quite a few options for consuming that data: build-in observer [0], CLI version [1], web live dashboard [3] and more.
[0] https://erlang.org/doc/apps/observer/observer_ug.html
[1] https://github.com/zhongwencool/observer_cli
[2] https://github.com/phoenixframework/phoenix_live_dashboard