I've had my Windows Phone for a few months now. In short: I like it. Unfortunately my support won't keep the platform alive. What works? What doesn't? What will it take to make it successful? Today: what works.
My phone: The HTC Arrive
The interface itself is easy to navigate. The lock screen is the first thing you see. In addition to the time, date, and day of the week, there are icons for missed calls, messages, and e-mails. It's convenient having this info available at the press of a button. Your home screen has live tiles (many of which give you info without your having to open the app) and the shortcuts you choose for easy access. One swipe over gives you a complete app list. The "people hub" links your existing contacts' information with their Facebook, Twitter, Windows Live, and LinkedIn profiles. It also gives you easy access to their latest social media updates and a history of all your interactions with that contact (messaging, e-mail, and calls). You can also separate people into "groups" but I haven't used this feature.
A Messaging thread
Messaging lets you switch between text messaging, Windows Live Messenger, and Facebook Chat, putting all three in one thread. If you use Exchange and Outlook for your e-mail, Windows Phone shines with fairly robust e-mail, calendar, contacts, and tasks synced through the cloud. I pay $6/month for Office365, which is essentially a hosted Exchange solution, but you can get similar functionality for free using a Windows Live account. The "Office hub" is a nice experience and gives you easy access to Office files on your SkyDrive (or SharePoint site). Office also gives you access to your OneNotes, which stay synced between the desktop, web app, and phone versions of the software.
Switch between recently used apps.
Hold down the back button and you can switch to the last few apps you've used (very webOS-like). Wireless syncing to your computer (with Zune software) is convenient and easy to set up via wi-fi. These are just a few highlights. I'm not going for product brochure here, just some stuff I think is cool. Plus there's the Xbox integration, which lets me watch my avatar dance when I shake my phone (not really important from a functionality standpoint but fun to watch).