Hm. They actually tell you what’s new in Leopard — from a Mac OS X developer perspective, anyway — in the release notes section of the ADC Reference Library.
Who would have guessed? I think I missed this for Tiger. And, AFAIK, it wasn’t available at all for any previous version of Mac OS X. ’Twas a shame, quite a shame.
(I’ve always missed John Montbriand’s old overviews of the Mac OS classic releases; they were a treasure trove, relatively speaking, of information. Even for sys admins.)