This is similar to what we're using at FastMail for our web interface, but simplified and made a bit more generic based on our experiences over the past year.
Our plan is to port our web interface across to this spec and implement both a direct backed in Cyrus IMAPd and a standalone implementation which can proxy for existing IMAP servers.
Keep up the good work! 6-year customer here. Notwithstanding some lost features compared to the old UI, your new web interface really lives up to your company's name. Everything is fast and snappy even when I'm accessing it from halfway around the world.
But I'm not sure it's a good idea to emphasize the fact that you use JSON. Sure, it's cool right now, but I find it technically more interesting that it replaces IMAP with something that is built on top of HTTP, a somewhat RESTful protocol.
You need some form of structure unless you want a zillion roundtrips. JSON is cool because it provides the basic structures (scalar, list, map) in a consistent and reliably parsable form. You don't need a context-aware 50 page ABNF to be able to parse it cleanly.
(don't even get me started on trying to write a proxy for IMAP that's aware of the connection state)
Our plan is to port our web interface across to this spec and implement both a direct backed in Cyrus IMAPd and a standalone implementation which can proxy for existing IMAP servers.