Some highlights:
New UI design
Self upgrade of both Showtime itself (only on PS3) and all plugins (all platforms)
Automatically load metadata from themoviedb.org
Improved support for subtitles (in particular external ASS/SSA files and vobsub files)
Support for on screen keyboard on PS3
Buttons on PS3 dual shock controller have changed their meaning a bit: See PS3 usage guide for details
Each focusable item have a dedicated popup menu (Use □ button on dual shock controller for this)
File browsing will sort folders first and then files. Sorting can be adjusted in the per page menu (△ on dual shock controller)
Lots of bug fixes
... + more stuff I've forgotten