Since I upgraded to Lion I am having issues with viewing QTs using Quick Looks. This is a feature that I use everyday in my line of work. If the file is an .m4v or is H.264 Quick Looks works fine and the video also shows up in Preview. If the file is an .mov or is DVC Pro HD (as most of my QTs are) then it will not open in Quick Looks and all I get in Preview is the QT icon.
Just came from the Apple Genius Bar. They said I may need to reinstall QT (which I just tried with no success) or do a clean reinstall of Lion (the last thing I want to do).
One other point is that I have QT7 as the default and lately QTX is not working at all, even if I change the default back to QTX. I also tried to delete QTX so that I only have one of them, but it wouldn't let me do that.
Other Lion users I know are not having this issue. I've been told I might have had a bad initial install. I installed Lion on the first day that it was available, but it has now been updated to 10.7.2.
If you can create a new quicktime file and it works with Quick Looks, then it could be an extended attributes issue. We use xattr to view the attributes of working file and broken files and we can usually change them. We've seen this impact a number of things including FCP project files (unknown file save errors for example).