May be Apple should rename "QuickTime Player" to "AV Foundation Player" - on Mavericks the "QuickTime Player" is not based on QuickTime but on "AVFoundation".
"AV Foundation" is Apple's replacement for QuickTime - QuickTime is not (and never will be) supported for 64 bit applications (and the "QuickTime Player" on Mavericks is a 64 bit application).
The "QuickTime Player" on Mavericks is converting all movies which are not H264, MP4, M4V (and may be a few more formats) because "AV Foundation" can only read / play movies with this format. Exactly the same as on iOS.
Apple's plan for now and the future: they expect you to convert all your movies. And trash all "old movie formats" (like AVI, DivX, WMV). If you have any "QuickTime VR" (panorama) movies: forget it.
It seems that Apple is ignoring that the users still need "old movie formats" and do not want to wait for movie convertion - my camera is recording AVI movies for example, and I will not buy a new camera just because Apple has this plan.
Fortunately Flip4Mac and Perian as QuickTime extensions still work on Mavericks if you use 32 bit applications. And Mavericks is supporting 32 bit applications. So this is what you can do: install Flip4Mac and Perian on Mavericks and use a 32 bit application like the old "QuickTime Player".