So you bought some music from an online site like WalMart's music store, or any other service that gives you a DRM-wrapped. Today, you have a license associated with your purchased music that tells Windows Media Player that the files are in fact yours and allows you to play them. Currently, you can move your license files around to any PC you own that you want to play your music on. It's a cumbersome method, but it makes it possible to share purchased music on a network.
Enter Windows Media Player 11, the latest and greatest from Microsoft in "We're not going to let you play that" technology. In WMP 11, you can't backup your licenses. If you want to play your music on another system, you'll have to jump through hoops set by whichever retailer you purchased the music from. And since each retailer can set their own restrictions on copies or number of authorized systems, you'll be left with a collection of music that has completely inconsistent rules wrapped around it.
Also, are you a fan of Windows Media Center Edition 2005? Do you use that system as a DVR for archiving your favorite shows and movies? Well, with WMP 11, you'll have 3 days to view the recorded content if it's flagged as protected content by the broadcaster.
These are listed as "Known Issues", but are they the sort of known issues that will see a fix, or are they simply acknowledging issues that are going to frustrate users? Microsoft is known for turning bugs into "undocumented features".