Hm, again. I don't think I agree it's that clearcut. I've found that if you mostly want a tree of simple key-value assignments, such as database.postgres.user

Hm, again. I don't think I agree it's that clearcut. I've found that if you mostly want a tree of simple key-value assignments, such as database.postgres.user or database.mysql.timezone, TOML is actually very hard to beat. It's extremely simple to teach to less technical coworkers. TOML mostly becomes somewhat strange if you need lists, or maps with free-form keys. Also, YAML is horrible if you have to generate it. Pretty much everyone automating stuff I've met will prefer JSON over YAML once you need to automatically generate the configuration. Pushing some data structure through an as-yaml pretty printer usually works, until it breaks in really weird incompatibilities between YAML parsers. And that happens way too much to be a fun distraction.