Leopard
Leopard is chock full of details like this—little things that won’t be promoted on the box cover or mentioned in mainstream media reviews, but which, taken cumulatively, epitomize why Mac OS X keeps getting better with each major release.
I don’t think there’s any one thing in Leopard, even Time Machine or Spaces, that’s a must-have for most users. Of the major features, Quick Look will probably have the most impact. Mostly, though, it’s the huge number of smaller improvements (for both users and developers) that make Leopard a great upgrade.
