I just got around to upgrading to VS2015 RTM having run the RC since it was available (just as an aside VS2015 has been the only version of Visual Studio that I've been running for quite some time now). And as Murphy's law goes, the first project I open doesn't build and run - OMG what pain do I now need to endure?
It appears that someone has made the decision to remove the Windows 8.1 Advertising SDK (perhaps due to the recent sale of MS advertising assets), which means my Windows 8.1 application is now missing a reference. According to the Ads in Apps website (http://adsinapps.microsoft.com/en-US/sdk) the SDK is installed in VS2013 update 2 or later (clearly not the case)
Luckily, a bit of searching revealed the SDK available via Visual Studio Gallery at https://visualstudiogallery.msdn.microsoft.com/3d3fa204-0641-4317-ab2c-50092f732edb