Like most of the developers who always like to run on latest and greatest Microsoft software, I too upgraded my machine to Windows 8.1 and VS 2013, Today I was bit disappointed to find out that I could not develop windows store app and target windows 8 platform. When creating new projects in VS2013 using the windows store project templates, 8.1 is picked as a default for me, there is simply no way to change this to 8.0 through the VS UI, I tried editing the csproj file and changing the target platform version from 8.1 to 8.0, while that seem to change the target platform after I reloaded the project it broke a bunch of stuff.
For now it seems like you have to keep both VS 2012 and VS 2013 side by side if you will be doing any windows store app development and want to target 8.0. VS 2013 project templates for windows store apps are designed to work with Windows 8.1 only. I don't like that I have to have two versions of VS in my dev machine but it is what it is. If you have any work around or suggestions, please post a comment