Well I've had an iPhone for over 2 years and the phone is great overall, but there are a few glaring problems that irritate me.
By far the worst is the lack of Flash support. Why Apple continues to refuse to support it is beyond me. This is the sole reason I will be replacing my iPad with another tablet when a great one comes out. Hopefully this will be soon since nearly every computer, electronics, or phone company is working on a tablet.
Next is the app store. Yes, there are over 300k apps, but 98% of them are garbage. It makes it tough to filter through the crap. A large number of these are also games, which I don't care too much about. I find that the best gaming experiences are had on a handheld or console with buttons and control sticks/pads. The games I play on my iPhone are simple, casual games. Based on my searches of the Windows Marketplace, WP7 will offer nearly everything I want and need for apps.
Another reason I've considered a WP is due to Nokia's decision to support WP7 exclusively. Nokia is still a global force, so the Windows apps will only grow more abundant when more and more phones sell worldwide. It's good for Microsoft to have the worlds biggest cellphone manufacturer supporting them