I think it's the very nice-looking and clean high-level API that should be a pleasure to use (when it fits the job, of course).

I'm pretty sure this API semantics (instance builder to configure, and then it's ask/paint/embed with language-native way to handle streaming and declarative tools) would look beautiful and easy to use in many other languages, e.g. I can imagine a similar API - save, of course, for the Rails stuff - in Python, C# or Erlang. While this level of API may be not perfectly sufficient for all possible LLM use cases, it should certainly speed up development time when this level of API is all that's possible needed.