SharePoint 2010 - What does a preupgrade check looks for ?

• Farm is at service pack 2 or later
• Supported operating systems (Windows Server 2008 SP2 64-bit or Windows Server 2008 R2)
• Database schemas have not been modified
• Configuration database does not have any site orphans
• Content database does not have any data orphans
• Farm is not still in a gradual upgrade from SharePoint 2003
• Databases are read only
• Invalid entries in web.config
• Custom site definitions
• Large lists
• Views and content types that use CAML