The reason Outlook is the world's most used mail client is because people are often forced to use it to access their company email. While anecdotal, I don't know a single person that uses outlook because they choose to. However, hopefully, outlook.com can introduce some innovation in the space, although, I'm skeptical as I am with most Microsoft 'innovation.'
I know a good few people that willingly use Outlook, including ones who use it for their Gmail account. Its actually pretty good at being e-mail PLUS calendar PLUS tasks, and so on. Google still haven't got that yet IMO, by virtue of "Google Calendar" and "Google Mail" being two separate products.
I didn't realize this myself, either, until I started working at an employer that uses the Google Apps stack for all things corporate.