Windows Phone 7,
Things just aren't going to work out for us. I don't even think you're a bad OS, you're just not on par with what I already have (Nexus One/Android). I wish you the best (and I'm excited to build apps for you), but you've got your work cut out for you if you're going to pull people away from iOS or Android.
First off, I get that you're selling the Windows ecosystem. I'm not interested (Gmail all the way here). You're going to need to streamline that initial signup a bit so that the first hour with the phone isn't a complete and utter buzzkill as users try to get ther contacts into place.
You need multitasking. Staring at a loading screen every single time I open an app is ridiculous. And to load again after swiping between pages makes it even worse. It makes the entire experience feel sluggish. Save state at the very least. Staring at loading graphics kills the joy of the beautiful UI.
Hubs are neat, but they get in the way. Sometimes I just want to find a phone number, and it's just clutter to have to wade through Facebook updates to get there. It looks great but I don't need a hub here, I want my information.
Your fonts are too big. As mentioned earlier, your UI is beautiful. The animations are great, the live dashboard is compelling, and everything feels incredibly polished (Android could take a few pointers here). However, your large fonts induce a lot of scrolling and it'd be nice to have more list items fit on the screen.
IE is bringing you down. Android browser is great, mobile Safari is great. IE is not. At all. Why do so many pages end up forcing a horizontal scroll? Ugh. This makes the browswer downright unusable. I'm fine with no Flash, but no YouTube videos at all? What year is this?
You need passive notifications. On my G1 (Android), there were apps that would cause an LED to blink in various colors to alert me to a new voicemail, email, or text. Even on the Nexus One the trackball blinks to alert me without needing to touch anything. Needing to turn on the phone screen is a minor inconvenience, but it does affect the overall experience quite a bit once you're used to that.
Those are the things that come to mind after a weekend with you. Sure, I want things like tethering and a good chat client but those aren't dealbreakers. The above are and they've sent me back to my Nexus One. You're the new kid on the block so I'll but you some slack, but you're going to have to do more than be pretty (oh so pretty) to win the day. Right now you're great for someone new to the latest generation of smartphones, but for someone coming from iOS or Android you leave much to be desired. Let's hang out in a year or so to see how you mature.
Image from Microsoft.