Microsofts superior UI design and integration of services makes wp7 the most pleasing platform to me. Even if all platforms had the exact same features, I'd pick wp7 for those reasons alone. Take into account the MS business & cloud services, tie-ins with office, xbox, windows 8 and its a no brainer.
The other thing I love about the platform in general is you have a highly polished, very professional product (like the iPhone), but you have choice in hardware. Because MS controls the minimum spec requirements, and has specific chipsets you must use, the experience on the phone will be the same as any other wp7 handset. But you can have the form factor you want. You aren't stuck with one design, or one model that just gets upgraded each year. Its like Toyota only making the Camry, and having a yearly model. What if you want a different style of body?
Different people have different needs, and having the ability to choose a form factor you want, while the OS & experience on each device having the same uniformity in UI design & performance is a huge to me. With the iPhone you have one model, one to choose from. And while its a nice peice of hardware, some people may want a keyboard, or a bigger screen, or a different type of screen, or a more basic phone for their kid ect.
wp7 was a big surprise to me, and I think MS took a big risk with a totally new UI design. And it works exceptionally well. It did take me a bit to get over my initial reaction to an interface so different from what I was used to seeing on everything else, but I cannot go back to the iPhone, or Android, or any other platform. I didn't even consider wp7 until 3 months after its release, and it took maybe 10 minutes of using it to be completely sold on its design and its future. Its that good.
Mango's already released features not only measure up to what iOS 5 is, it surpasses it in its quality of design, and the way it integrates everything into the OS and allows for a more fluid interaction without the up/down open/close app launcher system the iPhone has. I really can't say enough good things about the way the wp7 team designed this OS. The future is very bright for MS not only in mobile, but across all their platforms.