I have hand written a fair amount of json in the last week including some json schemas and the trailing commas gets me coming from golang literals and back.

I don't mind the idea of alternate syntaxes that compile to json in the spirit of coffeescript or less/sass/haml etc.

Coffeescript makes it very clear that it is just an alternate syntax for javascript. If that is the aim of json5 that is cool. I just don't want to see public APIs producing and consuming json5 in the wild and breaking stuff.