There are so many features listed on Apple's Leopard landing page that it might be easy to overlook this one (which Glyn Moody pointed out): OpenDocument Format, or ODF, support in the new operating system. It's baked right into OS X, and TextEdit will also support both Microsoft Word 2007 and OpenDocument formats.
At some point, Microsoft may also come around to ODF. In the meantime, there's Apple. Innovative as usual.
[UPDATED: As someone pointed out to me in an email, I made a mistake on "OpenDocument" in TextEdit. That appears to be a reference to Microsoft's confusingly named "open" format. But the ODF reference was right.]