According to the Apple Support note, in addition to Safari 5.0 and the improvements of the client version of the update, the Mac OS X Server v10.6.4 combo package adds Software Update and NetBoot/NetInstall support for the new MacBook and MacBook Pro models recently introduced, boosts reliability when looking up invitees in an LDAP directory, and "Improves reliability of Directory Services, particularly when Spotlight is enabled on the server."
Aside from the usual warning to never interrupt an update installation (or any install, really), here are the note's "read before proceeding" warnings:
The Mac OS X Server v10.6.4 Combo Update overwrites the /etc/rc.server file. If you have previously made changes to this file, make a backup copy before updating. Any custom sysctl settings should be configured in /etc/sysctl.conf rather than /etc/rc.server. See man rc.server and the /etc/sysctl.conf.default file for details.
You may experience unexpected results if you have third-party system software modiﬁcations installed, or if you've modiﬁed the operating system through other means. (This does not apply to normal application software installation.)
This update can be installed on non-startup volumes, but you should only do this if the current startup volume has already been updated to Mac OS X Server v10.6.4. The simplest way to avoid issues from an improper installation is to start up from the volume that you wish to update.