Edit from Jason:
"Please report any ... bugs you have with Leopard here. If you have more indepth questions, please feel free to post a new thread though. This is mainly for collection and reporting.
"
The short story: Two of five installs done in our household so far. The first one scared the hell out of us: crashed the hard drive on the iMac G5 (1.5 Gb), after two hours on the phone with Apple we took it to the genius bar, Applecare covered the cost of a new hard drive ($60 to transfer the data), tried the install again. HORROR SHOW. Time Machine wouldn't work, didn't see the Mighty Mouse, froze ten times a day, on phone with Apple for hours. Did every version of archive and install (preserve, don't preserve, etc.) and finally have Time Machine working, but forced to use a corded mouse - bluetooth is not seeing the mouse. Many, many lines of detail omitted, let me know if you want to know and I will turn my son loose on you for the gory details.
Next was my daughter's 1 Gb 15" iBook G4. It worked! One persistent challenge - doesn't seem to want to play videos in Quicktime. All she gets is the Q with a question mark blinking over it. Reinstalled QT and Macromedia player, no change.
REALLY not looking forward to putting it on my 1 Gb 17" Powerbook G4 currently running 10.3.9 quite happily. Waiting for 500 Gb external drive to arrive before I do that.
Then there is the 12" iBook G4 - not sure it can handle the onslaught of Leopard.
As for what is incompatible, aside from the bluetooth module, Mediafork keeps crashing while ripping DVDs. Previewing JPGs in full screen crashes the Finder consistently. Multitasking in iTunes crashes it, such as playing a song and deleting a playlist. Finder crashes daily, but it reboots in 10 seconds. And we can't seem to repair disc permissions.
Still beats the pants off the problems my neighbors are having with Vista.
23 years on the Mac and still loving it.