If this would ever have been released, it would have failed and die a painful death. As stated by the author, he "stole" (which means he faked) many UI elements from OSX because he "dislikes Metro". This is just wrong and bad practice when developing native applications. A UI designer should never design a UI for an OS which he/she dislikes to begin with, it will just end badly.
What makes Sparrow on OSX and iOS so good is that it stays true to its OS; that it feels native and blends into the OS with such grace that it's a thrill to work with. In contrast, web apps doesn't suffer much from this since users are used to them looking different, as long as they exist inside their browser environment. But for Sparrow to be successful on Windows, these rules would apply...
What makes Sparrow on OSX and iOS so good is that it stays true to its OS; that it feels native and blends into the OS with such grace that it's a thrill to work with. In contrast, web apps doesn't suffer much from this since users are used to them looking different, as long as they exist inside their browser environment. But for Sparrow to be successful on Windows, these rules would apply...