Winphone7.info has dig through the developer tools and has found several new items that were not previously discussed during the "Mango" event.
It is worth taking a look at the source link as there are many more updates not covered below.
One new additions is "Contact Card now has History pivot to see what communication you have had with the persons." This is a handy new feature that allows you to find your conversations by person, rather than having to open each individual communication tool and searching.
Another upgrade is to the maps section of Windows Phone. The 'Mango' update will bring maps that now show traffic flow and also a 'nearby' button that'll show you what's located around you. There's also an option to filter what is displayed when using the 'nearby' service.
You can view more of the hidden updates over here.