Microsoft also states that not all of the Windows 10 features will be ready at launch. As the summer turns into fall, more and more features will be added to the platform. Some features not expected to be ready at launch include extensions for the new Microsoft Edge browser, and a messaging app offering built-in Skype connectivity.
While Microsoft doesn't officially have a launch date for Windows 10 in mind, if it does get released this summer, there will be plenty of quick additions, updates, new features and more. The only thing that we can say at this point is that sometime before the end of the year we should see Windows 10 for Phone available for current handsets, followed by new Lumia models with Windows 10 pre-installed. Belfiore, the long time face of Windows Phone, notes that "Our phone builds have not been as far along as our PC builds. We’re adapting the phone experiences later than we’re adding the PC experiences."
source: TheVerge via MobileSyrup