One of my biggest problems with many Windows Phone apps is how much space they waste.
This is the official Facebook app from the Marketplace running on a Windows Phone 7 device on the right, and on the left is the Facebook app running on an iPhone. The yellow covers where the status information shows. It's hard to miss how much tinier the area is for seeing actual content compared to the iPhone, even with WP7's bigger screen. Really, Facebook?
And it's even worse: the Facebook app on WP7 does not follow my rules for hiding crap. What's hidden by the yellow in the picture here on the WP7 side are lots of stupid game requests and other garbage--things I don't see when viewing from either a computer or iPhone.
Those 2 things make the iPhone version so much more of a pleasure to use than the WP7 one.
Many of the built in apps do the same. I can see many more email messages at once on the iPhone compared to Windows Phone.
Our own new FTP On The Go for Windows Phone I think does much better. Still the Windows Phone style, but we really tried to also maximize the amount of space used for things useful for users--like folder listings rather than giant logos of our own. It does have a logo, but smaller and on the right, with the space on the left for the server, making double use of the same vertical space.
I didn't do a screenshot of the iPhone one, but it shows 7 1/2 lines in its folder view compared to the Windows Phone's 6 1/2. So a little less, but still good and fits the UI styles for both devices.