Well, there's definitely a good number of bugs with 8.1 right now, especially with the "newer" versions of apps - like Xbox Music and Video. Hopefully they'll have some updates shortly to help with that.
What phone do you have? I have no idea why your windows button isn't working half the time though - is it a capacitive button? And if so, does it vibrate when you tap on it? If not, it's likely a hardware issue - otherwise, hopefully just a bug that will be fixed upon the official release of 8.1.
As for accessing the file system, go download the "Files" app (was just recently released). You can manage all of your videos, documents, music, etc. from it.