I really like the OS and the social network integration is second to none. While it's lacking in apps, many of the major ones are there, but it still needs more developer support if it wants to starting picking up market share. I'd also like to see prices come down. iOS has set a precedent for cheap apps (games from major players and specialized software not included) and Android has most of its users trained into refusing to pay for software at all.
The real deal killer in WP7 for me was the email client. Something as simple as an email with 5 images attached results in an excessive amount of taps. Where iOS and Android display them inline, WP7 takes a different approach in the name of "security".
Using the example of an email with 5 images attached, here's how you view them:
Tap "Show all attachments"
Tap each image to download them
Tap the thumbnail to view it full size
Tap back to go back to the body
Tap the next thumbnail, etc
Total number of taps to view all 5 images on WP7: 15
Total number of taps to view all 5 on iOS (and Android, I believe): 1 (images are displayed inline)
If you go "back" out of your message to the inbox, when you click on that same email, you have to "show all attachments" again (though you don't have to re-download your images)!
On html emails with images, you only have to tap the button to show all images.