Posting a little tip that corrects a little annoyance with Mavericks and Yosemite. It is no longer possible to preview DNxHD Quicktimes (and all other QTs in codecs other than Prores ou H264) with the space-bar.
It turns out that the old behaviour can be recovered by "transplanting" the quicklook components that handle quicktime files from a copy of OS X 10.8.5. It works fine - although you need to redo the steps when you update the OS, and you need access to a machine with 10.8.5 to grab the files.
On a machine with Mountain Lion installed take a copy of the following two files/directories:
/System/Library/QuickLook/Movie.qlgenerator
/System/Library/Frameworks/Quartz.framework/Frameworks/QuickLookUI.framework/PlugIns/Movie.qldisplay
Put them safe somewhere (Avoid FAT32 formatted USB keys as permissions may be lost - zip them if in doubt)
On the machine with Mavericks/Yosemite, take a copy of the existing files/directories in these locations and replace them with those copied from 10.8.5. Be careful to only replace these files (not any part of the directories that contain them - doing so will create major headaches)
Run the following commands from the Terminal to repair permissions (you need to have a password set on the account):
sudo chmod -R a+x /System/Library/QuickLook/Movie.qlgenerator
sudo chmod -R a+x /System/Library/Frameworks/Quartz.framework/Frameworks/QuickLookUI.framework/PlugIns/Movie.qldisplay
sudo chown root /System/Library/QuickLook/Movie.qlgenerator
sudo chown root /System/Library/Frameworks/Quartz.framework/Frameworks/QuickLookUI.framework/PlugIns/Movie.qldisplay
Enter your password when prompted (nothing will appear this is normal)
Reboot.
The Space bar will now preview DNxHD files and QT reference files as before.
This should also work with El Capitan, but you will probably need to deactivate System Integrity Protection to replace the files (boot in Recovery mode (Cmd+R on startup), Launch Terminal and type csrutil disable), reboot, replace the files then you can probably reenable System Integrity Protection (csrutil enable in recovery mode) - I have not tested this.
Janusz