One of the most useful features of Windows 8.1 for me is the native support for Miracast (which is compatible with Intel Widi) for connecting to a wireless projector or display. Being able to wander around with my tablet whilst speaking is really handy.
Sadly, whilst this worked for a little while during the preview, everything stopped with no reason. Searching the internet hive mind suggested that a Windows Defender update during the preview release had borked it, but nobody could confirm.