> I feel bringing concurrency to Ruby (what Rubinius is trying to do) is going to be harder than bringing Ruby to concurrency (what Elixir is trying to do).

I don't really feel like the second of those is a good description, since it presents a false symmetry. Rubinius is trying to have solid support for good concurrency models in something that is actually Ruby.

Elixir is trying to have something vaguely Ruby-like that builds on existing support for strong concurrency models on the BEAM VM.

Obviously, the latter is an easier task, as it is less constrained (both seek strong support for concurrency, the former seeks being Ruby, the latter seeks only weak resemblance to Ruby), but the former leverages a greater subset of Ruby's existing strengths, including its existing ecosystem.