For taking care of the Year 2038 problem, OpenBSD decided to make time_t 64-bit for all platforms for OpenBSD 5.5+. The OpenBSD kernel plus all libraries and user-space programs were audited to ensure the 64-bit time_t support.
Other OpenBSD 5.5 changes include signing releases and packages using the Signify tool, installer improvements (including support for finally writing disk images to USB flash drives), many new hardware drivers, networking improvements, threading improvements, and a variety of other enhancements.
This is also the first major OpenBSD release that has Radeon DRM graphics supported ported from the FreeBSD kernel.
More details on the numerous OpenBSD 5.5 changes can be found via OpenBSD.org.