There will be a fair number of changes that comes with this update, where among them include new features such as photo library views, HTTPS support, Theme music as well as the ability to turn it off, the ability to skip forward 30s/back 10s during video playback, previous/next buttons on preplay, playlist preplay, play queue actions for music, mark season/show as watched/unwatched, delete items, user setting for blur/dim, and a single column view for library list.
There will also be bug fixes in this particular update, including grey/repeating posters in browse page, return to incorrect pre-play after navigating between episodes, unwatched/watched status of season/show may be incorrect, UI and icon tweaks. At the end of it all, the version number will be bumped to Plex 1.2.1.0.