I've been using HOCON for years.

Why it's not JSON:

* includes

* substitutions

Why it's not YAML:

* less confusing syntax (e.g. `cannot do that` doesn't need quotes, but `can't do that` does)

* built-in support for overrides via env variables

Why it's not JSON or YAML:

* Built in durations! No more head scratching to figure out if 5 is seconds or milliseconds. It's just `5s`.

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JSON is a data transfer format. YAML is a markup language (or it claims to be...).

HOCON is a composable configuration language. You always need to change, override, and deduplicate configuration in sensible way, in dev, in production, machine-by-machine.

Despite a wart or two I think HOCON takes a sensible to this ubiquitous problem.