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.