[virmundi]

Part of it is JSON as the storage format. Another part is its Node driver. The whole API fit it well. It understood async programming. It felt JS-like. The input and output were JSON instances. Finally, yea, from what I know of him "web scale" did play part in the decision. Oh those heady days.

[secoif]

For data that's mostly to do with the the API provided by the particular mongodb driver, than mongodb itself. Mongo stores and transmits BSON, not JSON. Most mongo drivers expose an API that serialises your data to BSON for writes and wraps the BSON data with a JSON-like interface for reads.

[sandGorgon]

postgresql has the jsonb column type which is just as powerful as mongo