Besides a new build number, we really can't spot the differences.
There should be some bugs fixed though.

This was wave 3 of Live Essentials. Wave 4 which is widely expected to be ready by the RTM of Windows 7, is already being tested internally, and beta invites are to be sent out in the coming weeks.

Multiple Points Of Presence Suport - Now you can sign into Messenger v9 from several different locations and remain signed in, In v8.5 and under, signing in at a new PC will sign you out anywhere else.
Signature sounds - Make sound signatures instead of just graphical ones.
Per contact sounds - chose a sound to associate with each of your contacts' actions.
Animated Display Pictures - now you can use animated .gif files as your display picture.
Links in Status Message - Links in the status message are now clickable
SPIM reporting - report and block users who spam you via IM.