It's not a new format, I have no idea why it's popular news today. Having worked extensively with both msgpack and JSON in a javascript environment, I can tell you, its the closest to JSON of all the binary formats. The difference with JSON is msgpack's strings are binary safe (you can have a png as your value) and the format is a bit more compact, especially around integers.
JSON has: numbers, strings, booleans, null, arrays, objects.
Msgpack has: numbers, raws, booleans, nil, arrays, maps.
So I guess msgpack is a superset of JSON. The raws can contain utf8 encoded strings like JSON mandates, or they can contain other things. There is no technical reason that the keys of the maps have to be strings. You could take a lua table that has another table as key and encode that in msgpack just fine.
In practice, I wanted more out of msgpack, so I extended the format using some of the reserved byte ranges to add in an "undefined" type and a distinction between utf8 encoded string and raw binary buffer.
For me this new format has been extremely useful as a general data serialization between processes (node to node, server to browser, etc..) I usually use it over binary websockets or raw tcp sockets.