I think Microsoft mostly needs two things for Windows Phone 8 to succeed.
1.) Great hardware partner. Nokia here, along with HTC and other little players.
2.) Great developer tools. We got Visual Studio covered here, along with things like Microsoft's XNA for games and easy, yet powerful languages like C#.
The idea here is that Microsoft really has all it covered. Nokia has a very stable history of making good phones. Their hardware really is rock solid. Nokia is the perfect partner Microsoft needs, and they have them. Motorola Mobility for Google doesn't even come close to what Microsoft-Nokia partnership is. I seriously think that Google tried to get Nokia on-board but they had already decided on Microsoft.
What comes to development tools.. well, you can't really go wrong with Visual Studio. It's an industry standard, really widely used IDE. Pretty much everyone agrees that it's rock solid product from Microsoft. Even if you hate Microsoft, you can but agree on this one. And the availability of things like XNA, C#, great documentation and the fact that Visual Studio Express is free really helps. Microsoft really is the developer friendly company. Much more so than Google or Apple.
I'd say these two things are well covered.
Then there's the matter of UI. Again, Microsoft has done remarkable job with the design. While I agree that Metro UI doesn't work too well on computers, it really is great on mobile phones and tablets. Everyone who has tested one of the Microsoft Windows Phone 7 phones can agree. The UI and system are good.
The last part Microsoft has in front of it really comes down to marketing. Nokia never really was that well known company in North America and that's why other companies like Apple and HTC have gained a following there. Nokia largely ignored NA market while they concentrated on Europe and Asia. Let's not forget that Nokia is still the worlds biggest phone manufacturer and controls almost half of the markets when dumb phones are included. Even without, Nokia has a much better base in Europe.
What Microsoft and Nokia need are phone companies that will push the products to consumers. That's all there is to it. They have a wonderful product in their hands but are missing the marketing required for it. I think it mostly comes down to so much different market than what it is in Asia or Europe. They just lack the experience.
Microsoft, or Nokia for that matter, could introduce one leading phone. The "one" phone that everyone would choose. But I think it's much better when Nokia produces many different phones and everyone can choose the one they like the best. Let's not forget that Microsoft does have hardware requirements so there is no problem with fragmentation like Android has. Apple, of course, has little next to none fragmentation problems, even with the different resolutions. Nokia and Microsoft are almost at the same boat.
All in all, both Microsoft and Nokia have wonderful product. They just need to market it to people.