Windows Phone had the best UX of any of the mobile operating systems, it was something I could hand to a user who'd never had a smart phone before, and it was much more intuitive for them - they could figure it out without help.

Sadly they never got enough market share (or perhaps investment by MS to pay for third party apps to get developed) to get the pool of Applications needed to attract users, which is unfortunate.

The one side effect of the 'easy to newcomers' UX, was experienced users had to forget a bit about what they knew about how a mobile device was supposed to work to use it - thats not a huge barrier, but I suspect it also made adoption by existing power users a little slower.