The WinME upgrade from Win98SE is totally pathetic. It often incorrectly copies over the dialup and network settings and corrupts those files, making it either impossible to connect to a remote server, or resolve domain names once connected. The only fix is to manually extract 5 files from the *.cabs into a temp dir, reboot with a startup disk, and rename the corrupted files. Then the new files must be manually copied into the proper locations. Total time speaking with a competent tech: around 45 minutes.
Clean installs of ME have been known to have corrupted dialup networking software, making it impossible to connect. The only fix I have found is to clear out the network control panel, uncheck dialup networking in Windows setup, reboot, and recheck dialup networking. Usually fixes the problem.
The system file checker has been replaced. It is still possible to manually extract files, but thanks to that oh-so-wonderful new "feature" called "system file protection", those files can only be replaced by booting with a boot disk. Not fun, especially with a user not familiar with DOS.
I hate the new shutdown options. That damn drop-down menu is so inconvenient.
For some bizarre reason, M$ decided to move Dial Up Networking out of the My Computer window. Everyone was used to that, and knew where it was, but M$ decided to change it.
I first tried the upgrade from Win98. It was a nightmare, locking up several times and giving several BSOD's. To be fair, though, I formatted and reinstalled without a problem and it's been stable ever since. I don't use a modem to connect, though, so that might be saving me some aggravation.