Can you explain why you think WP isn't a differentiator? It is clearly and significantly different visually from the other mobile os's.
I think this is more important to carriers though. They want people to walk in and see 5 or 6 different looking phones to choose from. Hence the love of Android.
There is nothing worse than carrier customisation - it's the worst kind of turd polish.
That lack of differentiation is murderous for OEMs - this alone is a brick wall against OEM adoption. The fact that OEMs have to pay to install this thing onto their phones is also not going to sit well at all, not when the competition is free.
The fact that it's a superior consumer experience won't matter at all.
Microsoft inevitably will have to start building their own devices a la the Lumia series, except they will have to really step up the game a lot more than they have so far. If I were MS I'd eliminate Nokia from the experience branding - their name would be on the box, but every slice of the out-of-box experience needs to scream Microsoft, and it needs to be comprehensively end-to-end. You use the Nokia name to make the sale, but the software should be 100% MS.
And for fuck's sake, they need to get their act together on the marketing front. We've seen precious little marketing for WinPhone, and worse, the little we've seen has been completely flat on its face.