For those not in the know, JMAP is a protocol for mail that, unlike IMAP, has threads and labels built-in at its core. In other words, Gmail without Gmail.
And it's about time somebody came along and built something sane for this.
There have been a lot of valiant attempts at this. Thunderbird has a few buggy extensions that attempt it. Notmuch is a very well written library and command line MUA with a very fast on-disk storage format (with ncurses, emacs, and vim interfaces, as well as neat FUSE file system toys), but it is not a protocol, and therefore does not scale well beyond the limited set of MUAs for it that mostly need a shell or a flimsy web app. Mailpile is an attempt at making a Web 2.0 style Gmail clone, but it's only an MUA and the on-disk format is flimsy at best.
JMAP defines the protocol first. Then, multiple people can take a stab at the most sane implementation. I like this approach quite a bit.