I have been using smartphones and tablets a lot for the recent years, including several years using Windows Phone and a year using a Windows 8.1 tablet: the Dell Venue Pro 8. I would say that Windows 8.1 works great as an OS for both consuming and occasionally producing content on the tablet; however, it is not excellent as an OS for connected consumer devices.
Below are several things that I hope Microsoft will sort out before the launch of Windows 10 next year. Those are also the features that have not been introduced or those that I have never heard any rumor about on Windows 10, so Action Center (coming for sure) or Cortana (definitely coming sooner or later) will not be included here.