QuickTime 10.0 was first demonstrated at WWDC (Apple Worldwide Developers Conference) in 2009. The application ships with Mac OS X 10.6 and higher. This version of QuickTime was rewritten from scratch to include support for 64-bit codecs, which in turn does not allow the player to natively play some legacy codecs, such as MIDI, interractive QuickTime movies, and QTVR.
QuickTime 7.7 can be installed alongside for legacy codecs playback according to various internet sources (however, I failed to install this version alongside X version). Currently, Apple is offering a developer's preview of the newer version 10.1 with additional sharing and fast forwarding features. The update will be available through Software Update Center when it officially comes out. Though, there is a strong possibility that this version will only be available for Mac OS X 10.7.
QuickTime X is a very simplistic universal media player with a minimal set of features. Videos can be trimmed right inside the program. The trimmed file can be converted for playback on your Mac computers or your portable iDevice. The files can be also exported directly to YouTube or uploaded to your space on MobileMe cloud service. All of those features are available right from the main playback window of the player.
The player supports resizing of the playing video, including Full screen mode. The player does not have preferences dialog, therefore no way to customize the player through the traditional way. There is a movie inspector included with the package, which displays the relevant information about the file that is being played. Export manager shows you the status of your currently exporting file. Overall, this is a really unloaded application by Apple. The company had concentrated on getting the player to do what it was intended to do - play media files and they've done an outstanding job as it plays media files.