Apple’s document on Extending and Troubleshooting Directory Services has a lot of good info.
One correction though is that the debug level must be an integer.
sudo defaults write /Library/Preferences/DirectoryService/DirectoryServiceDebug "Debug Logging Priority Level" -integer 2
I’ve notified Apple, so this may be fixed by the time you read this.
Update: That link is dead. Here is an article that offers some similar information.