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Finder has, in the process of going 64-bit, been rewritten using Apple's Cocoa API, although you'd never notice from the UI which is, on opening, exactly the same as before. In use, however, Finder fits better with the rest of the OS, rife as it now is with Core Animation (although actually that get's old fast). The ability to resize any icon up to 512 x 512 pixels would seem pointless, were it for for not for icon previews.
A fairly logical extension of Quick Look (which let you open files without launching a dedicated program), icon previews enable the viewing and playing of content from within the icon itself. It worked with absolutely every file I could think through, an MP3, a 1080p movie trailer (the playback of which is impressive in and of itself) and a PDF (through which you can navigate the pages) all previewed in their icons with nary an issue. It's not exactly life changing, and it's usefulness is fairly limited in all reality, but it's still pretty cool nonetheless.
Another improvement comes in improved notifications on trying to eject USB storage devices. If you try to eject a usb drive while it is still in use the error message thrown up will now say what program is accessing it, along with the option to force eject anyway. Frustratingly Apple still doesn't seem to grasp the concept of 'plug and play' as simply yanking a flash drive out - even if not being accessed by any program - still throws up an error.
The Dock is now getting in on Exposé, too, with the aptly named Dock Exposé. Click and hold on an application icon in the Dock and Exposé will pop up, but only for that app's windows. It's especially useful for dragging between application windows, rather like in Windows, funnily enough. You can also right click application icons to see a list off all the open windows.
Exposé now also let's you order windows either alphabetically by title, using Command+1, or by program (and then alphabetically), using Command+2. I'd argue that if you have enough windows open to find that useful, you'll probably have them organised using Spaces, though, so it'll be of niche appeal at best. More usefully, windows are divided between active and minimised in the Exposé view.
Stacks are now improved, too. Clicking on a folder within a stack now opens its content in the stack, rather than bringing up a Finder window as before. Further, in grid view stacks with more content that can fit on one page can now be scrolled - why that behaviour wasn't present originally I don't know.