JSON-LD is JSON for linking data and it's a web standard: http://json-ld.org/. It offers a smooth, even zero-edit in some cases, path for upgrading existing GeoJSON data and makes web-friendly extension of GeoJSON possible.
Previously, if you added a "timestamp": "2014-03-27" to a GeoJSON feature object's "properties", a user has to guess what you mean. Playing guessing games with my kids is fun. Playing pointless guessing games with data sucks. One of the most important things JSON-LD offers is a framework in which you can unambiguously identify "timestamp" as "http://purl.org/dc/terms/created". Now users of your data have a solid start toward understanding what you mean by "timestamp".
If you'd like to help shape the way we use GeoJSON in the future, please follow https://github.com/geojson/geojson-ld and jump into discussion on the open vocabulary and context issues.