WP7 phones have rather tight hardware requirements [1] that to a degree influence the general handset. I fail yet to see how Nokia can top that (Maybe coming up with a better Camera/Zeiss optic. Maybe putting quite a lot more power in the handset - but the first option would be more for a niche imo and the second would cut into margins if you want to be competitive).
Sure, it's all just speculation until we see their first devices. But at this point I remain unconvinced that Nokia can differ enough to dominate the WP7 platform.
There are plenty of other perks MS can offer Nokia too. Windows Phone 7 ads that prominently show Nokia phones being the most obvious. This is likely worth hundreds of millions of dollars in its own.
I suspect Nokia will continue MeeGo development in case the Windows deal fails, so they have an alternative platform to fall back on. The potential for Android-compatible MeeGo handsets being rolled out at a moment's notice should keep MS in check.
Nokia wouldn't be a minor player in the Android ecosystem. That's ridiculous. If they executed quickly and release some good Android phones within a year, they'd become relevant again. With their power, they could even beat Samsung, which is going to become the biggest Android phone manufacturer.
Of course they'd still need to compete well hardware wise. This decision seems to me like they need software differentiation because they know they can't compete well enough on hardware anymore.
By the time Nokia makes a few WP7 phones and sells them, Samsung will sell a lot more Android phones. I expect Samsung to become the #1 phone manufacturer within 2-3 years. This won't happen only because Samsung is growing, but because Nokia will sell a lot less Symbian units, due to competition in the mid and low end from Asian manufacturers.