To give a counterpoint, I not only know maybe a half dozen people who voluntarily use Outlook; I know at least two who voluntarily use Outlook on a Mac.

I think the fact that few developers enjoy using Outlook is causing you to overgeneralize. Most people who don't care about things like whether their email is in mbox format, or who don't want to learn how to use Mutt or notmuch, find Outlook surprisingly easy to use and powerful.

I poked around on outlook.com, and I don't see anything particularly innovative, other than it looks and works nearly exactly like Outlook 2013. But I wouldn't assume its lack of anything novel dooms it to obscurity, either. Outlook, unlike Notes, is a positive brand-name in some circles. I wouldn't be surprised to see this get a relatively large amount of traction right off the bat.