Windows mobile (Windows Phone 7) is so new, but Microsoft is announcing the second generation for this year. We know about Microsoft / Nokia agreement, but the Nokia sites still shows Symbian powered phones.
Windows Phone Second generation
It seems to be that Nokia is waiting for the second generation of Windows phone that according to Steve Ballmer (MS CEO) will be available by the holydays time, he said that with this new platform users will be able to:
do things they had never imagined before.
Skype and Microsoft
Maybe one of the best advantages Microsoft has now, is the acquisition of Skype, this means that Windows Phones may be able to "do things never imagined before", they can integrate Skype with other applications like email and messenger.
In fact if the have good imagination they can get some benefits of this.
And there is Office 365, yes there also be iCloud and Google Docs for iPhones and Android but most of the users in the world use Microsoft Office, even Mac users load MS Office in their computers.
So, if you really want to have full compatibility you'll have to get a Microsoft phone.
In their page the say Office 365 is available for all platforms:
Work from almost anywhere and get automatically updated email, calendar, and contacts on the devices you use most, including PCs, Macintosh computers, iPhone, Android phones, Symbian phones, BlackBerry smartphones*, Windows Mobile, and Windows Phones
But they may add some features only for their phones.
I think Microsoft will use its big users base, (Windows users, Office Users, Exchange users) to promote its phones, otherwise it will be difficult to compete.