Well, I'm not sure about codecs, etc. but there is definitely a difference between the players in QT 6 and 7. The main thing is that QT 7's player is *finally* Cocoa, and in turn is much faster and more responsive. It also has a nice, cleaner design that matches the system better:
Also, QT 7 Player holds good news for those who use ShapeShifter/system themes - QT Player now uses normal brushed metal, so when you switch themes it'll still match the system. Since it's Cocoa, you can also now mess around with the NIB files and rearrange the interface elements, as well as turn off the metal look, which is very cool.