The particular implementation I had in mind is Erlang. Due to the green thread implementation, immutable data, and message passing, Erlang is able to offer a simple, robust concurrency platform. Of course it's still possible to write very sequential code but generally someone writing idiomatic Erlang won't have to go out of their way to get concurrency.
It is also far from ideal when it comes to performance from what I've seen. Maybe its overall design is good, but I don't believe that adding so much overhead over native programs is necessary.
From my somewhat limited reading it seems that the only inherent performance hit in Erlang's implementation is that all its processes can be interrupted externally, I think the term is cooperative multitasking?
Otherwise I think the rest of Erlang's performance is because it's optimized for reliability and distribution rather than raw performance.
