Version 11 features a host of new features which are mainly focused on user interface enhancements and online music sharing/purchasing,information retrieval etc. The interface looks cool and is in league of Windows Vista Aero UI. It only occupied 12MB of memory (with no file being played) and it loaded in few seconds in our crappiest PC which is a 256MB RAM/ 1.5Ghz P4 when we ran it for a test run. That's pretty good performance for such a powerful application. We wouldn't go through all of WMP11 features here because it would turn our blog into a Microsoft advertisement.
However there are little annoyances with this version too. Some users may dislike the new UI and this version seems a little less customizable than version 10. But there will be apps released in the future that would enable tweak it to user's liking. These are only minor problems.
This REQUIRES GENUINE VALIDATION. When you download the setup and run it, the first step in setup will instruct you to go get yourself validated :p If you say no, setup will give you the finger and quit.