The IIS 7.0 health model (Windows speak for known diagnostical and failure conditions experienced by software) has just been pubished on TechNet. You can get to it here: http://technet2.microsoft.com/windowsserver2008/en/library/b19873a2-9f72-40c8-b73d-89f39cda62781033.mspx?mfr=true. Credit goes to the hardworking people on our product support and documentation teams.
The health model is primarily a list of event log events that are raised by various IIS subsystems, including WAS (technically WPAS now, for Windows Process Activation Service), the IIS worker process, Web Publishing Service (W3SVC), etc.
The coolest thing about the doc is that it provides a detailed set of steps needed to resolve the problem. No more wondering what the hell the famous "The data is the error" IIS-ism means 🙂
You can use this information to diagnose and resolve error conditions that result in entries in the Windows Event Log. These are typically process or service level events, so this doesnt help with a lot of request error conditions. Even so, its great info to reference when you are experiencing system level issues. Enjoy!