The widespread usage of JSON as a configuration format is disappointing.

JSON is intended to be and is primarily advertised as a lightweight data-interchange format. And for this use case it really shines. However as a configuration format it falls short on multiple aspects, compared to YAML, which I highlight below (examples taken from Learn X in Y minutes ):

• Indented syntax reduces unnecessary boilerplate:

a_nested_map:
    key: value
    another_key: Another Value
    another_nested_map:
        hello: hello

• Inclusion of larger blocks of text is not awkward:

literal_block: |
    This entire block of text will be the value of the 'literal_block' key,
    with line breaks being preserved.

    The literal continues until de-dented, and the leading indentation is
    stripped.

        Any lines that are 'more-indented' keep the rest of their indentation -
        these lines will be indented by 4 spaces.
folded_style: >
    This entire block of text will be the value of 'folded_style', but this
    time, all newlines will be replaced with a single space.

    Blank lines, like above, are converted to a newline character.

        'More-indented' lines keep their newlines, too -
        this text will appear over two lines.

JSON in contrast does not have support for multiline strings.

This is particularly useful when configuration files are being used for internationalization.

• Anchors to reduce duplication:

# Anchors can be used to duplicate/inherit properties
base: &base
    name: Everyone has same name

foo: &foo
    <<: *base
    age: 10

bar: &bar
    <<: *base
    age: 20

• Native Support for Dates and DateTimes:

# Strings and numbers aren't the only scalars that YAML can understand.
# ISO-formatted date and datetime literals are also parsed.
datetime: 2001-12-15T02:59:43.1Z
datetime_with_spaces: 2001-12-14 21:59:43.10 -5
date: 2002-12-14

Plus many others.

Also it is noteworthy that YAML is officially a super set of JSON so migrating from JSON to YAML is effortless.

Most of the times I use plain text or JSON. Simply because is all I need for my use cases. In situations where things get more complicated, something else may work better, but for now, plain text works just fine. If it is in PHP, I may just have a config array that may get filled from a text file...