As you already know (if you don’t, go read about it right now), IIS7 core server has been completely re-engineered to provide a brand new extensibility model on top of which all of the server features are built. This allows anyone to remove/replace all existing server features, or add new ones - by using either the new native APIs, or the managed ASP.NET APIs.
Over the past two years, we did a bunch of work to make sure the new native APIs are easy to use and help write more robust server code. But, it is hard to appreciate all the benefits until you actually compare writing an IIS7 module with writing an IIS6 ISAPI Extension – I am often amazed at how nice some of the little improvements can make your life.
Read more here - http://mvolo.com/2006/10/07/10-reasons-why-developing-server-components-with-the-iis7-native-api-is-10-times-much-better-then-iis60-isapi.aspx.