Spacebar in Quicktime Player X used to pause video, no longer does in OSX Lion
The problem is if you use the two fingered gesture to fast-forward or rewind the video.  As soon as you do, you'll notice that the tab focus jumps to the position indicator.

Here's a normal Quicktime Player control before using gestures:


Here's what the control looks like as soon as you use a fast-forward or rewind gesture:


Notice that the diamond position indicator is now blue.

Seems to be a particularly annoying bug in the player.