JSON configuration rather than YML
I have had so many issues with configuration due to the unnecessary imposing and contradictive whitespace rules of YML.
For instance, having to indent with exactly two spaces, apart from when the section is a named list, in which case you use four spaces.
For someone who writes code in a language that uses braces to indicate their block level, which is the same case for the structure of JSON, using YML feels worse than writing XML by hand.
JSON is very popular and I can't see a reason as to not use it for configuration, at least as an option.