Office Web Apps can be used only by SharePoint 2013 web applications that use claims-based authentication.
Office Web Apps rendering and editing will not work on SharePoint 2013 web applications that use classic mode authentication. Hence, something to watch our when migrating SharePoint 2010 web applications.
Do not enable minor versioning in document libraries that contain OneNote notebooks. Minor versioning can result in synchronization errors that prevent edits from being saved.
Default crawled file name extensions and parsed file types in SharePoint Server 2013
User permissions and permission levels in SharePoint 2013
Create and deploy a thesaurus in SharePoint Server 2013
Software boundaries and limits
This will be a work in progress posting, as I'll add when I encountered them.