The new features include:
– Windows Live Messenger: Windows Phone 7.0, surprisingly, does not support Microsoft’s consumer Messenger instant messaging or Communicator/Lync business messaging services. There is no indication so far that Lync will be supported in 7.5.
– Facebook Chat: Microsoft provides a free Facebook app that does not support chat
– AOL Instant Messenger (AIM)
– Group SMS (text messaging)
– Office Hub SkyDrive integration: Currently only OneNote integrates with SkyDrive
– Office Hub Office 365 integration: Office 365 Exchange services such as email, contacts, and calendar currently work with Windows Phone 7. Other components do not
Via winrumors: More Windows Phone 7.5 features leak, Facebook chat built-in