The biggest change at this moment is "Digital Media Mall", which is a bunch of links to webservices selling music, like Napster and MusicNow.
A feature which is not yet fully available is the ability to automatically synchronize or transfer media files between computers and portable music or video players. Microsoft hopes that this easy-transfer feature will help the sales of a new line of portable devices that are based on Microsoft's Windows Media Center software.
Also embedded in Media Player 10 is much of the code for Microsoft's latest copy-protection system, which will give media companies greater control over how often tracks of music can be copied and transferred to other devices. That will include tracks rented from subscription-based services, which currently can be played back only on Windows computers.The beta version of WMP10 can be downloaded here
Source: Washington Post