Hey BaSsie86, I'm glad I could help!
I did further testing and I was wrong on my previous post. Perian doesn't have anything to do with it, sorry for the misconception. What does affect the video corruption is the kernel and 32-bit/64-bit modes.
So my results were:
10.7.4 - 64 bit kernel mode - mov videos are OK, Quicktime player and thumbnails/preview play without corruption; mp4/h264 videos corrupted in thumbnail/preview and in Quicktime. *
10.7.4 - 32 bit kernel mode - mov videos are OK on Quicktime but not in thumbnails or preview; mp4/h264 videos corrupted in thumbnail and Quicktime.
10.7.5 - 64 bit kernel mode - mov videos are OK, Quicktime player and thumbnails/preview play without corruption; mp4/h264 videos corrupted in thumbnail/preview and in Quicktime. [same as with 10.7.4] *
10.7.5 - 32 bit kernel mode - mov videos corrupted on Quicktime player and on preview; mp4/h264 videos also corrupted in previews and Quicktime, only the most common codecs work without corruption in previews/Quicktime.
So, 64 bit kernel mode is definitely better for overall video compatibility, although some h264 videos will always be corrupted under Quicktime and thumbnail/previews. But it will bring the Opengl graphical bugs back, so no 3d gaming on 64 bit kernel mode. One has to choose. As far as I'm concerned, 64-bit kernel mode seems to be the solution since I'm not using this machine for any gaming, for that I have my Windows7 machine.
*Also, the video file extensions can be changed from mp4 to m4v or avi or mkv to fix the thumbnail/preview. We just have to avoid using Quicktime on those files. Any 3rd party player, like Movist or VLC or MediaplayerX works 100%, so no problem at all!
And this is enough troubleshooting for me, I'm happy now with my system since everything is "workaroundable". Thanks everyone for their info and help, I couldn't have figured this out with the testing already made by clemj06, theconnactic, ptp, etc, around pages 44 to 60.