The IIS 7 configuration system contains 50+ configuration sections (100+ if you count .NET Framework configuration), 477 attributes, and 57 collections. This can make it pretty challenging to figure out exactly what IIS 7 configuration section needs to be used to turn on some particular behavior, and then exactly syntax the configuration in that section should follow.
This is not typically an issue for me, because I can write most IIS configuration from memory. But, what if you didn't spend the past 4 years developing IIS7?
Learn some tricks for demystifying the IIS7 config at http://mvolo.com/blogs/serverside/archive/2007/11/01/IIS7-configuration-sections-exposed.aspx.