Some simple thoughts:

I feel that one challenge of programming languages is how to remember these rules, formats, and keywords. Even if you're using familiar formats like YAML or JSON, how do you match keywords?

When developing Glicol (http://glicol.org/), I found that if it's based on an audio graph, all node inputs and outputs are all signals, which at least reduces the matching problems. The remaining challenge is ensuring that reference documentation is available at the minimal cost.