The concurrency model. I recommend Rich Hickey's talk "the language of the system" (ironic, I know). In erlang/elixir you use the same kinds of abstractions to design your fine-grained logic and you coarse services. This makes for a very unique approach to scaling.

I'll have to check that out. Thanks for the reference

Elixir (probably due to the Erlang roots) seems to actively make it hard for you to hold onto state. This ends up (in my experience) being a good thing... or at least a brain-stretching thing that may lead to fewer bugs on larger projects.

Clojure allows some OO if you'd like. I see that as a drawback, personally, but I've drank the functional koolaid.