Earlier this week, my wife upgraded her PowerBook to Tiger. Big mistake. She found herself unable to connect to our wireless network at home. Even worse, while her machine was trying to connect, it prevented any other machine from connecting. It appears that something about how Tiger requests an IP address causes our US Robotics 8054 router to continously reboot itself. Other people have reported a similar problem with a D-Link router - supposedly the problem has been fixed in the forthcoming 10.4.1 update. A work around that solves the problem in the short term is to use a static IP address on the Mac instead of a dynamically allocated address.
The other problem that she encountered was that her Epson printer stopped working - it displayed a cryptic error about the raster imaging system. It is apparently quite a common problem with the Tiger upgrade - removing the printer and re-installing it seems to fix the problem.
At least I know that even if I switch my parent's computers to Macs, I'll still be needed for tech support duty. ;)