The following changes have already occured in OpenBSD 5.5-current and will be part of the November 1st OpenBSD 5.6 release:
ALTQ traffic shaping removed from PF. No more oldqueue tricks, you will have to move to the new system.
sendmail will be removed from the base system, smtpd is the default mail transport agent (MTA). The legacy sendmail MTA will be available as a package.
Kerberos has been removed from the base system. Kerberos support will be available via a package.
The BIND (named) name server will be removed from the OpenBSD base system. Its replacement comes in the form of the two daemons nsd(8) for authoritative DNS service and unbound(8) for recursive resolver service.
The base system httpd, a forked Apache 1.3, has been removed in favor of nginx as the system HTTP(s) server. Apache 1.3 will be available as a package.
The installer no longer supports FTP, network installs via HTTP only. If you run an internal mirror, your life just became easier.
OpenBSD 5.6 will be the first version with LibreSSL (OpenBSD's cleaned-up fork of OpenSSL). No user-visible changes are anticipated.