Microsoft did a nice job in reinventing its mobile OS with the advent of Windows Phones. There are ways in which the "Metro UI" is beautiful and strikingly different than iOS and Android. However the UI also represents the greatest barrier to adoption of Windows Phones.
Until people have a chance to use and familiarize themselves with Windows Phones they won't buy them because of the perceived unfamiliarity. Agressive discounting in the US may convince some to do so however. But as long as there are relatively few Windows Phones "out in the world" the impulse to buy them will also be limited.
It's Microsoft and Nokia's version of the "chicken and egg problem." Targeting new smartphone owners is probably the best strategy to gain share for the companies in the near term. Once they have some users they can upsell them to more expensive and high-powered phones.
Among developers however, Appcelerator has discovered that there's an increasing appreciation for the Metro UI. In a survey of more than 2,000 mobile developers, 44% of them characterized the metro UI as "different and beautiful" compared with iOS and Android.
That 44% hasn't yet translated into a belief that Windows Phones will succeed -- even in the enterprise, where Microsoft has had historical advantages over competitors. Developers in this survey do think that BlackBerry is essentially dead in the enterprise (which would mean death for the company overall) and thus by default that Windows will be the "third ecosystem."
Speaking purely for myself, the Windows Phone homescreen is a barrier to adoption. While the "live tiles" are supposed to facilitate quick access to content and enable us to "get back to our lives" I find them awkward and off-putting. I think the modifications of Windows Phone 8 make it worse.
The "inside" of Windows Phone is much better and more pleasing. Regardless, I think the outlook for Windows Phones (and by extension Nokia) remains very mixed at best. Unless or until Microsoft and Nokia can get these phones in people's hands they'll have trouble winning share.