[fcbrooklyn]

The real "killer feature" is the Erlang VM (the BEAM). Thanks to a combination of exceptionally lightweight "processes" (not true OS processes, a construct of the VM), which share nothing, can only interact through message passing, and which can be supervised by other processes, it's possible to build incredibly robust systems.

Elixir makes the Erlang VM much more pleasant to use in my opinion (plenty of Erlangers will disagree with this, ymmv). It provides a developer-friendly, modern ecosystem, with doc support, testing support, a great macro language for building DSLs, package management, etc, but underlying it all is a battle-tested VM, that has been under active development for 30 years.