Well, I've had quite a few problems so far:
Startup time has increased two minutes (now 7 minutes, 5 with SP1, 3 with XP Release)
Security Center apparently drank some molasses, the menus are very slow to drop down/go up
It decided to turn off ClearType, so I wasted a good two hours looking for the pref to re-enable it
As Antony mentioned before, it killed ZoneAlarm, but I found out that it just no longer likes to boot with the system, in which case I now start it manually.
Accessing the Internet is now a joke, see the SP2 kills ZA 5.1 thread for all the ZA prompts I got.
Start menu has become very unresponsive. I usually have to click it once, then it will pop up and disappear two seconds later, click it again and it will be stable there. After that, if I modify a start menu item (or an All Programs item) the start menu will collapse and I will have to repeat the process.
Right-click yields the same problem as the Start Menu.
For all we know, we could have WinXP SP2.1 at some point.
UserAgent: Mozilla/5.0 (Windows; U; Win98; en-US; m18) Gecko/20000929 Netscape6/6.0
Laptop: HP Compaq nx6325 - Turion 64 X2 @ 2GHz, 2GB DDR2, 100GB HD, ATI Radeon X300, 15" LCD, Seven Pro
Handheld: Palm Treo 650 - Intel PXA270 @ 312MHz, 10MB RAM, 32MB flash, 2.7" LCD, Palm OS 5.4