10
I don't know about 10 gotchas but here are a few that got me:
1) Performance. This has biggest one for me so far. It depends on the app but it can really be hard to get right on some apps (especially data heavy applications). Things get worse where pivots and panorama's are involved. Data binding and XAML based templating all have a performance cost which is magnified on a small device. The biggest problem I see in this regard is people bringing "skills" over from the desktop. People new to mobile development often make the mistake of applying enterprise design patterns and frameworks to device programming. Be very weary of bloated frameworks and lofty design patterns, such things are designed to solve the problem of complexity in large applications and aren't necessarily helpful on small projects like these. You don't need a whole framework to do MVVM.
2) Device incompatibility. With only a few WP7 devices out there currently this isn't too big of a problem yet, but there is one place I have had problems - Samsung SuperAMOLED screens. These screens really show up the limitations of a 16-bit color depth display, much more than a standard screen. It can lead to really horrible looking color banding if you are not careful with your colors and gradients. I've blogged about it here (with an example).
3) Tombstoning. Depending on your app this can either be a breeze or a real hassle. Don't make the mistake of leaving it till the end of your development to test. Try and think about it upfront.
4) The Marketplace. Register early. Unfortunately, IMHO the Windows Marketplace is the weakest part of Microsoft's app story for developers currently. More than half of the developers I have been involved with have had problems getting registered (including myself). It is very convoluted and seems to go wrong ver