The request for comments on QED went out last September and the QEMU Enhanced Disk format can already be found within the QEMU 0.14 release. This new format tries to be superior to QCOW2 and others. Current features include fully a-synchronous I/O path, strong data integrity, backing files, and sparse files.
Being worked on QED for the future is image streaming, online defragmentation, TRIM support, parallel submission, and meta-data scan avoidance.
Additional details on the QED disk format can be found on the QEMU Wiki.