I find Factor to have a compelling sweet spot of concise syntax, dynamic features, and (relatively) high performance. But in particular, I think we've done a good job with a few things:

1) Making the language very "clickable", so you can introspect and dig down into all function and type definitions.

2) Provide a lot of batteries-included in the standard library.

3) Make it super easy to accept contributions, there's a part of the distribution that is "extra" and has low barrier to entry and then we commit to keeping code working and promoting things to the main standard library when they are useful/documented/tested enough.