There is a Marketing your app section in MSDN, where you can find information about how your app will appear in the Windows Store. But you can find nothing about what your app needs to get into the Spotlight section on the first page of the Store.
Well, that’s intentional to prevent gaming the system and abusing the algorithm. Wouldn’t you just buy your own app ten more times just to get into Spotlight, would you? For this reason don’t expect Microsoft publish the algorithm…
Some tips that can help your app:
Make sure you provide Promotional images in the Description step of the publication wizard, because without this your app has no chance to get into Spotlight. You can upload PNG images in four sizes:
You don’t have to upload images in all sizes, but the more you upload the more chance you have. If you want to upload only a single image, let that be in 414×180 pixel size.
Second, make sure your app is polished. Even the simplest app has more chance to the get into Spotlight if you care about the details when you make it, than a more useful app that look like you spent no time on it. Don’t even hope with the default X icon:
Finally, follow the feedbacks. You can get feedbacks from your users not only through the Review page, but the Store also collects statistics about your app, which you can review on the Dashboard.