Something I particularly like is the philosophy of not forcing you down any particular design path, but supporting as many as possible. For example, if you happen to be using the Model View Controller design pattern, GWT would be brilliant for creating the “view” part, and you could use something else for the Model and Controller parts. Or you could not use Model View Controller at all. Also, you don't have to use the whole toolkit, you can use just parts of it, the components are designed to work independently of one another.
I'm still not sure that “AJAX” web applications are the best solution for software as a service in the long term, but they're certainly the best technology currently widely supported, even if you have to deal with browser quirks to ensure compatibility.