One of Microsoft's evangelists working with students put together this list of common certification issues.
- Make sure all your views work. On Windows 8, this means snapped and filled views along with portrait. On Windows 8.1, this means handling variable view sizes down to 500px, and possibly to 320px if you set that flag in your manifest.
If you use sounds in your app, make sure there's a clear way to turn them on and off, or set the volume (between 0 and full).
- It's possible that the manual tester might not see the option in your Settings, so you might need to put a control on canvas if it won't get in the way (and that button can open settings, of course.)
- If you have ads, you need a privacy statement in your Help or other settings where the tester can see it.
- Make sure to pick your countries carefully: there are cultural differences that will allow/disallow various metaphors and imagery. The author of this list said that one of his apps with "kisses" in it was disallowed in some countries.
- Age rating: be sure to study the requirements here, as this is a common stumbling point.
- Never, ever upload an app with the default Visual Studio logos and splash screen. Hopefully the WACK is catching these by now, but I don't want to see any more apps in the Store with the default graphics!
Test on different devices and in the simulator
- If the simulator hangs on launching your app, there's a definite problem.
- Use the simulator to test orientation changes and all view states on all canonical screen sizes.
- If you have access to different devices, including Windows RT devices, by all means try them out there too. You'll get a various of scaling factors and screen dimensions.