If Windows 8 was touch first, then 8.1 Update is most definitely keyboard/mouse first. Almost every single feature added in the Update is geared towards making the Desktop environment a first class citizen again, and it’s wonderful – and I say that as someone who uses and enjoys Windows 8 and 8.1 every day.
Boots into Desktop by default and Modern apps can be run windowed. If you use Windows on a non-touchscreen device, this is a good update. Oh, and Universal apps – one app with binaries for Windows Phone and Windows 8. Windows on ARM devices gets a bit of love too.