As has been discussed on Apple's Web site, as well as here on MacFixit, many users who upgraded to Quicktime 7.4 lost the ability to view mp4 video files - only the audio content would play. Unfortunately, this problem has not been addressed with v7.4.5.

For the people who will invariably chime in with "Just use VLC" - we know. Using VLC doesn't change the fact that Quicktime is broken and no longer works as it should (and previously did).