Great things about Windows Phone 8.1:
Calendar is truly great now with detailed week view, for me the best improvement so far.
Start screen, I try to keep it tidy, so for me more space for tiles is not really that necessary, but I do like the background-picture feature and that when using the “show more tiles”-mode you can make the tiles smaller.
And of course the action center and notification bar. One of the features I missed from Android.
And, is it only me or is it really just a bit faster?
I won’t write any big review or walkthrough on this, other people have already done it for example: