The only new feature in the beta is the option for vertically orientated tabs, which a number of people have been requesting for quite some time. A movie showing what the vertical tabs look like is also available here.
Changes from 1.4.7:
- (New) Support for vertically oriented tabs on the left and right sides of message windows
- (Bug) Fixed a problem where tab animations became stuck when opening or closing a tab
- (Bug) Tear-off windows are now always the same size as the window they are removed from
Update: I initially uploaded a version where non-English localizations did not have the new vertical tab options added. Redownloading 1.4.8b1 will solve this problem.