This quick post is for my own sake so I can find it again later. :-) I refer to the following link often.
Mike Volodarsky has put together a detailed list of breaking changes in IIS 7.0 and ASP.NET. For the most part IIS 7 has great compatibility with IIS 6 apps. The httpModules and httpHandlers section are two important web.config sections that need to be migrated from system.web to system.webServer in Integrated mode, but most other things will work without changes.
There are exceptions though, which his blog post details well: http://mvolo.com/blogs/serverside/archive/2007/12/08/IIS-7.0-Breaking-Changes-ASP.NET-2.0-applications-Integrated-mode.aspx