In any user-editable text, comments is useful both to explain sections, and to temporarily disable a section. I've been looking for a new generic data structure representation to replace YAML, and JSON seemed like a really promising candidate, until I found out about this limitation.
*Update: The plot thickens. According to the JSON Wikipedia Page, the JSON spec used to include support for /* comments */ (c-style). Seems they changed it. Conveniently, this helps to make JSON a YAML subset. I'm pretty sure it is not worth that tho. Anyone have a link on the process that resulted in this?*