From Apple’s support forums, the top user gripes about Mac OS X Snow Leopard.

1. HP Printer Problems - Error #9672 appeared for dozens of users attempting to connect HP LaserJet P1006—and other printers—to their Snow Leopard Macs.
2. Bricking and the Spinning Wheel of Death - If you’ve tinkered under the hood of your Mac and install Snow Leopard, watch out. The forum regarding these crippling faults stretched for miles.
3. Incompatible Applications - Amazingly, the Mac’s anti-virus apps don’t work.
4. Cicsco VPN - The solution here is to contact your IT department to get Cisco VPN reinstalled on your computer. Also try a complete reinstall.

These are major issues if they affect you, but they’re minor issues overall.