What has to stand out is the OS and the ecosystem, with solid hardware. Right now if you do not like iOS, your choice is Android.
Android has many issues from fragmentation, to very inconsistent user interfaces that vary by brand, to an app store with regular malware culling, to battery life issues, to out of date builds with no updates. To say there is not room for another solid competitor is simply wrong. While Android is growing like crazy, survey data is suggesting people are buying on price or dislike for iOS, and are not that in love with the actual OS (the dreaded "would you buy again" question).
WP7 brings a lot to the table. Perhaps the best OS of the three? The answer will depend on personal taste of how you expect your OS to function, but certainly has a very well thought out and original interface.
Microsoft is building an ecosystem that ties into their Xbox platform, and entertainment platform that Android lacks. (Outside of Amazon's and similar, which is why they will succeed).
It uses windows development tools that are very solid, and have an army of windows developers. iOS and Android developers will not have terrible difficulty porting their apps. Microsoft has the resources to develop apps equal to or better than Googles (except search *cough*).
Many of the Advantages of Android over iOS will likely not exist with the WP7 ecosystem. For example a wide range of hardware and multiple price points. Most likely companies will build one device, and sell models with both OS's.
They are just now shipping competitive hardware, and the OS is really just now starting to mature. Sales will take off soon.
Just a quick non researched reply, but obviously WP7 has enough compelling reasons for it to exist. While I likely will not buy one, I wish them well, competition is good.