I'd rather have trailing commas, so every line can have the same format. Other people choose JSON for their config file format, and I don't have a say in it. I just have to do extra editing each time I reorder the lines or add a new one, because too bad. And I have to manually edit JSON when I'm writing code to handle other people's JSON APIs to test my code.

I'll probably never not have to manually edit JSON, so JSON's insistence that using the same format for your final line as for all the others won't ever be allowed will remain a never ending, unnecessary nuisance in the spirit of JavaScript itself: the "too bad, too late now" school of design. Except that in this case, even JavaScript corrected that particular nuisance and allows trailing commas.