It's designed well, from the ground up with a lot of lessons learned from some pretty serious email junkies. Anyone hemming and hawing about JSON being selected as the transport encoding or whatever is just raising silly points. You could do JMAP over protobuf too probably if you cared, but the whole point (IMO) is to get the hell away from unstructured protocols.
Yep, there's always someone ranting about JSON inefficiencies, which I don't observe at all :)
oh i mean i rant about it too but its like, a byproduct of the real goal, and since most clients are running some type of webapp it makes sense. you could definitely use something like messagepack or protobuf if you had an extension that negotiates it.