When in column view in OS X, the final column is designed to present a 'preview' of whatever file is highlighted in the previous column - text, image file, whatever. I've found that its behavior in the PB is somewhat unpredictable - sometimes you see a preview of an image file, sometimes you don't; text files seem to work regularly. Tonight, though, I noticed for the first time that Quicktime previews work really well. If you highlight a Quicktime movie in column view, you get a mini Quicktime player in the next column with audio and video controls. Easy movie browsing without launching anything!
When this feature is ironed out in the final release, it could be one of the nicer usability touches in the OS.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010201223710340