Here's the old (2012) HN thread with the haters in it:
https://news.ycombinator.com/item?id=4031699 . Happy to see an old (positive) comment of my own that I had forgotten about, which I think sums the whole thing up quite well. It still applies, so I'll repost it here:
You know that something has gone very deeply wrong somewhere when people oppose your project because they are ideologically opposed to comments. In my own experience, finding out that many JSON parsers reject comments was a real WTF moment, and a deal breaker for my config-file application, so I ended up using JSON-plus-comments for it instead of JSON. At the same time, lack of support for trailing commas and unquoted member names have been a minor but persistent thorn in my side for no good reason.
The justification for not having comments in JSON is that in the great disaster that was XML, some projects would parse the comments and take them as semantically significant. However, the real problem there was that parser libraries would expose the comments, and that some generators would put important information only in comments. But I think that these mistakes are unlikely to be repeated, and that the proposed alternative - moving all comments into the markup, or eliminating them entirely - is just obviously worse.
One of the things that ruined the XML ecosystem was a persistent belief that XML was to be read and written by machines, combined with a reality in which it was mostly used for human-written config files, leading to a tolerance for awful syntax (like prefixing every single attribute with a namespace, and the ridiculous CDATA notation). I'm seeing the same thing with JSON: A significant fraction of its use is for human-written and human-read config files (which often desperately need comments) and people are pretending it's strictly a data interchange format that shouldn't be used for that.
It is sometimes said that JSON was discovered, rather than invented - that the syntax was already out there. So it is with JSON5: There is nothing new in this, it is simply return to JavaScript Object Notation and bringing in the rest of what Javascript has.
So, all you pooh-poohing ideologists: please seriously rethink whether disallowing comments, trailing commas, and quoteless member names is actually a good idea. Consider this in light of the fact that JSON is widely used, today, as a config-file language, and that {"--":"This is a comment"} is ridiculous enough that no one does it in practice, can't be inserted on any line, and invites consumers of your data to parse the comments.