Many times in running their Macs people will install various drivers and system enhancements to customize their Mac experience. These can be anything from a device driver to a haxie for enabling hidden OS X features. While these may work when running your system, there are times when updates to system software can break their function or cause other unexpected behavior. Usually these issues are not bad, but periodically people may update and wonder why they are suddenly getting errors or hangs.
One possibility for this is if your third-party drivers are not being properly updated along with OS updates. Apple's built-in Software Update service allows you to quickly apply patches and updates, and while this allows many people to keep their operating system at the latest version, it does not provide an update notification for drivers or other third-party software.
As a result, many installed drivers may be out of date, so be sure to know which you have installed and check with the manufacturer for updated versions. Some will continue to be compatible with the latest OS version, but others may be updated to fix issues that crop up with new OS releases. Some drivers have included system preferences or other settings that include their own automatic updates, but this depends on the specific software package.
Usually after updating (though preferably before doing so), be sure to check for compatibility with your third-party add-ons. These can be found in the system menu, or in the "Other" section of the System Preferences, or in the various parts of the Software section in the System Profiler utility. When listed, they may show as being supported either by Apple or by a third party" which will indicate whether Apple will be updating it or if you will need to check for updates through other means.