The first of the big announcements for the user end is Mango will be bringing messenger to Windows Phone 7, which will support groups and threaded chat. SMS conversations will also receive this threaded feature as well as the ability to group texts. Allows integrated communication to friends without running applications for specific tasks. Windows Live will see a more prominent presence.
Also on the groups note, Live Tiles for individual contacts will notify the user upon changes with alerts for new emails etc. from that specific contact.