As per Windows 8.1 Update: Sideloading Enhancements

  • As part of Windows 8.1 Update, we are enabling sideloading for all Windows 8.1 Pro devices that are joined to an Active Directory domain, matching the current behavior of Windows 8.1 Enterprise. So if you are using either of those two versions, you won’t need to worry about sideloading keys or activation at all.
  • As of May 1, 2014 customers in the following Volume Licensing programs (regardless of what product they purchase) will be granted Enterprise Sideloading Rights and provided with a sideload key at no additional license cost.
    • Enterprise Agreement
    • Enterprise Subscription Agreement
    • Enrollment for Education Solutions (under a Campus and School Agreement)
    • School Enrollment
    • Select and Select Plus
Doesn't address the almost complete unavailability of .appx packages for paid apps, or the plethora of other serious problems with the Windows 8 app ecosystem, but it's a step in the right direction.