I spoke too soon about no problems with Leopard. I forgot one major part of my setup - my Epson R1800 wide format photo printer.
I went to print a photo for a Christmas present and found ... no printer. Installing Leopard had silently removed my Epson printer driver. (The CUPS + Gutenprint driver was still there, but I only use it to handle printing from Windows under Parallels.)
I can see a driver not being compatible with a new version of an operating system, but to just silently remove it seems pretty lame. Ideally it would warn you at the start of the install so you had a chance to abort the upgrade if you wanted. At the least it could notify you that it had removed your printer!
Luckily, I had waited long enough to upgrade that Epson had released new drivers. (They were released on Dec. 18 - if I had upgraded a week earlier I'd have been screwed.)
All's well that ends well - my printer is working again and I got my Christmas present done :-)