The problem with an app store for a windows PC, is how often would someone even need to use that? The only types of programs that people are buying on a regular daily/weekly/monthly basis is games, and Steam/Uplay/Origin already has that angle well covered. Outside of that, what software would people buy for their PC? Anti-virus? Thats a one-time purchase that you never need to make again. Office suite? Same thing. Tax software once a year? Productivity/business software isn't something that people are going to be buying on a frequent basis at all. I just don't see a lot of point to a Windows store for PCs.
I dont see the Windows store every becoming very useful unless they start selling media (music, movies, Tv shows), but you're going to have a big problem convincing people to switch over from iTunes or Netflix or however they currently get their media.