Leopard was a fairly large change as classic (Mac OS 9 and earlier) was dropped from this release. This also was the last release that Power PC (PPC) based hardware can run.
Notable new features are Spaces and Time Machine. Also some important security frameworks debuted on Leopard: Application sandboxes (for system apps), application code signing, and the firewall transitioned from kernel based (IPFW) to a socket-level application-specific firewall.
Leopard was also "late" as it was initially described semi-publicly to developers in Mid 2005 as being targeted for early 2007 or even late 2006, but it was not shipped until October 2007. Apple stated that manpower was intentionally shifted to the iPhone which was released in the time between when Leopard was announced and shipped.