I've mentioned previously, there has been some experimentation around with getting WebKit to work on windows. I just didn't really expect Apple to release a full Safari for Windows. So naturally owning a MacBook that I use to run both OSX and Windows, I downloaded both versions. Although the Windows version gives you a taste, it just doesn't 'feel right'. Not sure what it is, but Safari just seems so much smoother in it's native OSX. I'm sure future versions will improve on this, but wow, cool.
Another feature of Leopard that I'm pretty impressed with is the ability to have 'Piles' or 'Stacks' of icons and files. This is not entirely a new idea, but I'm not aware of a decent usable implementation that has full integration into a consumer desktop. Knowing Apple, it's probably pretty smooth with all the appropriate transition effects and bling.