"must haves" may be strong words (ava0775), but I understand the things he mentions as being deciding points in selecting a phone OS. I'm on the other side (Mac OS-X and iOS) but appreciate how an Office user would want his/her phone to intergrate seamlessly with a desktop or laptop pc. My current experience with a Blackberry, even now after the very long awaited BB for Mac client, is that calendar and contact synching are problematic; easily as many error mesages as successful synchs.
The peace of mind, not to mention time and trouble saved, of knowing changes made to a calendar or contact informaiton on the phone will be reflected on the desktop (or an admin assistant's pc) is huge. I know this can work from my iPod and iPad experience and I am loath to forego it when i next upgrade my phone. If I were a PC, I'd expect as much of Windows Phone 7.