Mobile Review gave a very detailed 13 minute walk-through of the upcoming Windows Phone 7.1/Mango. This is the first very deep glimpse into what the OS update will look like and it shows the platform reaching a major maturity point. With this update Windows Phone 7 is looking less like a 1.0 product and more like a mature competitor to existing Mobile Operating Systems (i.e. iPhone and Android devices).
There are a host of features that will be useful to users. Some announced, such as Linked Inboxes, and some not yet seen, such as Contact Groups or Multi-Tasking Views. I recommend you watch this video for the whole run down.
Source: Windows Mobile Power User via Mobile Review.