Even casual users find typing faster than mousing. So while there are modes of interaction where it’s nice to sit back and drive around with the mouse, we observe people staying more engaged and more focused on their task when they can keep their hands on the keyboard all the time. Hotkeys are a sort of mental gymnastics, the HUD is a continuation of mental flow.
It’s smart, because it can do things like fuzzy matching, and it can learn what you usually do so it can prioritise the things you use often. It covers the focused app (because that’s where you probably want to act) as well as system functionality; you can change IM state, or go offline in Skype, all through the HUD, without changing focus, because those apps all talk to the indicator system. When you’ve been using it for a little while it seems like it’s reading your mind, in a good way.