When submitting your application to the Windows 8 store, you'll have to comply with these certification guidelines:
Make sure your application passes and specifically watch out for these:
4.1.1 Your app must have a privacy statement if it collects personal information
4.5 Your app must protect customers from unintentional large data transfers over metered networks
Is your app downloading large amounts of data? You're required to show an indication that the user is on a 3G connection (metered network) and that he might be charged for data usage.
You can check the connection type of a user with this line of code:
var networkCostType = NetworkInformation.GetInternetConnectionProfile().GetConnectionCost().NetworkCostType;
6.2 Your app must have a Windows age rating, and you must submit third-party ratings for your app if you have them
If you want to make sure your app passes, choose a high age rating. You might think your app is suitable for 3+, but the app might load content you don't have control over (rss reader, streaming video's). Or submit (personal)data to a webservice.
An age rating of 12+ will do the trick to get your app certified.