64-bit time_t: Yes, we're year 2038 ready, but this means the format of all binaries changes. Upgraders must remove and reinstall packages, and certaing data files such as the password database needs to be generated. (Also, our msdosfs aka FAT file system support breaks in 2100 CE, see http://marc.info/?l=openbsd-tech&m=139971926513249&w=2 for details).
New traffic shaping - prio and queue (ALTQ kept around for one release only, grace period via the oldqueue trick)
Much improved autoinstall(8) via dhcp config and response file
Releases, packages and patches are now signed using the new signify(1)