Microsoft's latest developer communication newsletter sent out to the community revealed a few new features:
In the email, Microsoft stated that Mango would also bring "support for real time communication and messaging and a broader connectivity to Skydrive." It sounds Microsoft will be implementing OS-wide chat (this could be Windows Live Messenger or even Facebook Chat), as well as utilizing the "Messaging" hub for all aspects of communication rather than just text messaging. The update also is likely to implement better support for accessing files already stored in Skydrive.
Also, that "the new Windows Phone Marketplace experience will include an improved design with panes and more panoramas, a better search experience with autocomplete queries and search results grouped by app, music, or games sections. A new jump list and a soft-search button will make it easier to find installed apps on Windows phones."
Microsoft also noted that the Windows Phone Marketplace will extend from 16 to 35 countries in the new release, including Brazil, Chile, Colombia, Czech Republic, Denmark, Finland, Greece, Hungary, India, Japan, Netherlands, Norway, Poland, Portugal, Russia, South Africa, South Korea, Sweden and Taiwan.
Also note, diTii recently reported that "Mango" update would be called Windows Phone OS 7.5 officially, and it would be coming with multitasking support, IE9 + full HTML5 support as well as Twitter integration and massive improvements for developers.