We actually offer both for configuration files of our software - YAML can be very pleasant to write and read for small datasets or those without much depth, without the overhead of curly brackets and quotation marks on everything. The original problem these formats solve is that people just want a simple nestable key-value structure/format, and both deliver on it - unlike xml. (when do you use an attribute vs a tag and text element?)

Parsing JSON is a minefield: http://seriot.ch/projects/parsing_json.html

Its spec is not "complete and unambiguous"!

Not quite unambiguous! It declares how to parse numbers, but of course the "reference implementation" of JSON (insofar as there is one) is Javascript, in which numbers are floats and hence e.g. are bounded in precision, so the spec actually disagrees with the reference implementation.

And trailing comments?