Q: Quick time will not play videos on Lion - why?

What type of files are you trying to open?

What file extension?

wont play any for me neither, tried DivX and .avi

also finding the media centre wont open neither, have to listen to music etc through itunes, which isnt as convenient.

Ditto - just upgraded my wife's iMac and Quicktime is hosed up.  Previews don't work on .mov files and Quicktime hangs when trying to open the same files.  I have tried numerous .mov files that used to play before, but will not play now. 

I will be upgrading my MBP tonight and will see if it works....

Same here, already started a thread:

https://discussions.apple.com/message/15714124#15714124

Seems like ProRess for me is not previewing. Some files are though.

oh yeah, just did a full recovery (not clean install) and nothing has changed. So dont waste your time. Might have to look at  a clean install.

well this seems to have fix my problem.

- Uninstall any Divx components. I had 3vix installed, once removed it all started working.

good luck

I had the same problem in Aperture not playing or recognizing my .mov files. I found a thread that suggested I uninstall 3ivx which I did. It fixed the problem. But aperture still doesn't play my Nikon P7000 movies. There is an uninstall 3ivx if you search in spotlight you should find it.

BINGO

https://discussions.apple.com/message/15715508#15715508