I just installed the 'Mango' update to my Windows Phone 7.
It is definitely the largest update to the phone. A lot of the hubs have been cleaned up and look better. Screen navigation through the main phone pages is quicker (they seemed to have shortened the navigation animations).
The office hub can now search and find all the OneNote 2010 folders you have in the cloud and will sync them all.
The marketplace looks cleaner.
You can add twitter and linked.in
accounts (personally I don't think I'll bother)
There is more than just English input! My partner has had to use a special app for the last 9 months to send SMS in chinese. She'll be able to do it natively through the phone now.
The people hub does have a new feature to create 'groups'. But I don't mass message anyone.
But MOST importantly, emails are now threaded! Finally it looks clean and collapsed like gmail.
My only gripe with Mango? The updated Internet Explorer doesn't have a button to quickly access tabs any more. It is hidden off under the '...' extra options.