The Mac OS X Finder should catch up to and improve upon the thumbnailing functionality of KDE’s Konqueror and GNOME’s Nautilus. I’ve previously stated that such thumbnails are of limited use, but that will change as screen resolutions and DPI counts rise.
The movie portion of my mockup assumes support for icons larger than OS X’s current maximum of 128 x 128. There is supposedly already some support for 256 x 256 icons in Mac OS X, but I don’t know of any application using this size or of a way to scale existing icons to 256 x 256 through the GUI.
For consistency’s sake, the zooming animation used when opening and closing files should be of the thumbnail, not the generic filetype icon as is currently done. Try double-clicking a thumbnailed image (generated by the Finder, not set as a custom icon) in Icon view to see what I mean.