Wait until Microsoft spend $500 million on a WP7 exclusive app...
Well, maybe not $500 million, but Office for WP7 will be better than Office for Android or iOS equivalents, even if it means MS have to buy those developers first :)
I don't see that reason yet either, but after playing with a Nokia Lumia I do believe it's within the realm of possibility.
They did get the basics right here, WinMobile is actually surprisingly good. First and foremost it's hands-down the most responsive UI, even an iPhone feels laggy in contrast. Also most of the core interface and features worked really well for me when I tried; the whole thing feels right, the interface is intuitive and beautiful. It seriously makes the iPhone look dated.
What it lacks (apart from bugfixes) is customizability and the app ecosystem. Those are obviously huge issues, but not insurmountable when you have the warchest of a Microsoft.
I'm not seeing a similarly good reason to buy a WinPhone.