Even though the app has a lot of cosmetic improvements such as rotating backdrops, more artwork used etc etc, the biggest changes are under the hood. Playback is vastly improved and performance is even better.
Here's a list of the biggest changes
Playback
Support for HLS streams, providing a more reliable transcoding solution with less playback errors.
Support for external subtitles without transcoding
New player look and feel
Features
Channels are now supported. Just enable some channels on the server and enjoy them on your windows 8.1 device.
Playlists are now supported, just go into the music hub and you'll find them.
Multiple backdrops. The app now use all the backdrops that you have for your items and fades nicely between them
Fewer buttons, the resume button is gone and replaced with a flyout. Just hit play, if the playable item can resume the options will be shown
The change user option has moved up to the top left corner of the main screen
Languages
Two more languages have been added, Spanish and Kazakh