OpenBSD 5.5: The new traffic shaping system (to replace ALTQ, which goes away totally in 5.6) with a new, friendlier syntax. Filtering with received-on any now possible. The block policy in the default pf.conf is now block return.
OpenBSD 5.6: ALTQ removed from OpenBSD (even the oldqueue transition trick).
(Also, OpenBSD IPv6 configuration defaults changed: -inet6 is the default, IPv6 addresses (including link-local) set only when explicitly enabled, global accept_rtadv sysctl removed. No PF syntax change needed though.)