Apple made a dumb argument. The point here is that "App Store" explains what the service is or does. "Windows" does not explain what the OS is or does, and nor does "Office." This isn't about a word that's commonly used, but about a word that explains what the actual product is.
No way is Apple winning this. Dream on fanboys.
You can get descriptive trademarks, i.e Vision Center. And again, the whole mark counts in the evaluation. There are as of today 686 APP trademarks at the USPTO, including Apple's App Store. But besides all that, you're going to have a difficult time showing that consumers would be confused. Apple owns the public perception of App Store. That's a steeper hill than you think.