Windows Phone (especially after the 7.5 "mango" update) has received positive reviews -- some extremely positive. I have one of the Samsung Windows Phones and have been using it periodically for several weeks and can attest to some of the accolades. The browser for example is very fast. I also like the operation of email on the handset.
Some of the various reviews assert that the Microsoft OS has "caught up" to Android and iOS. And with the release of the first Nokisoft phones, the future looks much better for Microsoft's mobile efforts (and maybe for Nokia) than it did a year ago.
However, right now at least, the best prospects for Windows Phones are feature phone users upgrading to a smartphone. These are people who generally speaking haven't been living with and become "acculturated" to an iPhone or Android device -- although the iPod Touch and iPad expose non-smartphone users to iOS.
There's almost zero chance that an iPhone user is going to switch to a Windows Phone at this point. By the same token the chance that an existing Android user will switch is low, though not as low as with the iPhone. iPhone owners display greater loyalty than Android owners. By my logic, then, Windows Phones are most likely competing for attention from those individuals considering upgrading from a feature phone to an Android handset. (Although the iPhone 4 is now $99 with a contract.)
In addition, Windows Phones are probably not competing with the top-of-the-line Android handsets (e.g., Samsung Galaxy, HTC Rezound, Moto Razr). They'll be competing more at the entry level, although Nokia's Lumia handsets are not positioned as entry level devices. To win buyers, however, they'll need to be priced as though they were entry level smartphones.
Windows Phones must generate sales to show that the platform has traction in order to make their case to developers. Without developers and a sufficient supply of desirable apps, Windows Phone will remain a second-tier OS. For higher-end users, Microsoft also needs to "answer" Siri with some compelling voice capabilities in future updates.
There's no word on precisely when the new Nokisoft handsets will be available in North America. They'll come first to Europe and then perhaps Asia and the developing world before the US market. This makes sense because Nokia's brand is much stronger in Europe and developing markets, where the company is known for cheap devices.
To compete in the US these Nokia handsets -- and Windows Phones in general -- must be priced at or below $150, and probably $99, with a two-year contract. That's chiefly because they don't have the apps ecosystem to compete with Android. Until they do this is a major deficiency and competitive disadvantage. That's why price is key. But Microsoft knows all this.
If I were a US-based marketing executive for Windows Phones I would secure carrier relationships that allowed pricing at $99 (with a contract). Then I would target smartphone upgraders (low-end Android buyers) and make the case that the Nokia-made devices are better.
I would also be very aggressive with developers. For example, I would pay them to port over their most popular apps to Windows, which appears to be what Microsoft is doing. And I would allow them to keep 100% of the proceeds of app sales for the first year on the platform -- maybe the first two years.
All of this positioning advice is just based on my instincts and market observations (rather than survey data). But Microsoft and Nokia need to "get it right" or potentially miss a window of opportunity. And without some initial success and perceived momentum, both developers and carriers will be less interested going forward.