My smartphone OS history has gone from Windows Mobile 6.5 (HTC HD2) > Android 2.3 (Nexus One) > iOS 5 (iPhone 4) > WP7 and Windows Phone has been my favourite (still rocking my Radar as I await WP8). For me the lives tiles are a big feature I enjoy. I hate to sound like an ad, but the glance and go is perfect. Although Android had widgets, it looked ugly to have a bunch of widgets on my home screen since they all didn't follow one design guideline.
Also, the fact that so many features are built in (local scout, bing music, social networks, etc) and don't require an app download makes it easier to access these features as the OS places them very intuitively for easy access.
The fluidity of the WP OS is the third big feature that keeps me sticking in the Microsoft camp. Although iOS is very smooth too, the design is so boring. The live tiles may be a simple design, but it just looks so nice. I have an iPad, so I still get my iOS fix. But for my phone, I definitely plan on upgrading my Radar this fall to a shiny new Apollo handset.
Lastly, it's the little features and the fact that Microsoft paid attention to the little details that I enjoy. It's small things like the lock screen bouncing, or the signal strength dropping down when you open the phone app, or the live tiles flipping away when you launch an app that makes me appreciate the effort Microsoft put into this OS to make it so great.