I think it's great because it extends .NET with the same API's and the same Developer Tools. It's great because it has lots of end user functionality out of the box for developers to extend. It's great at being an Office content server and integrating with Word, Excel and other client applications. It's great because it offers an enterprise capable web application development environment.
My opinion, maybe I'm biased, but this guy doesn't work for Microsoft. He's got the right idea: