Multiple Apps Snap Views
Microsoft made extensive changes to maneuvering multiple apps on the screen and it seems responsive based on your screen size. Not a bad job at all.
Internet Explorer 11 has unlimited tabs and a reading list
They seem to have done a better job of refining Internet Explorer as well. It seemed smoother and more polished which is a very good thing.
Skype login from the home screen
This was discussed in the Windows 8.1 announcement at BUILD 2013. Microsoft are stating that the Camera and Skype will work right from the login screen without you being logged in.
That seems like a really cool development as well.
Multi-Monitor Support in Windows 8.1
In Windows 8.1, multi-monitor support is greatly improved.
You can resize apps to nearly infinite sized windows, share the screen between two apps, or have up to three apps on each monitor.