Mango is still quite immature compared to iOS 5 or Android ICS. A little detail I notice today in the Calendar app. Swiping to the left or right did not navigate to previous/next days/months. Lots of obvious missing features like that in Mango. Hopefully Apollo fixes it, but the average consumer has no true compelling reason to buy a WP instead of an iPhone.
There are plenty of reasons for an average person to buy WP7 instead of an iPhone.
Here are some reasons someone might not like the iPhone
1. 3.5" screen
2. No LTE
3. Grid of apps makes the baby jesus cry
4. Hate iTunes
5. Don't want to worry about holding the phone wrong
6. Dual glass sides have a high rate of breaking
7. One form factor
I had an iPhone 4 and while it was an ok experience, it failed in a few areas. First, the reception was not that good and I dropped calls frequently. Second, it took too long to do stuff since to do anything I had to open the apps. Third, the 3.5" screen is just terribly small.