Leopard will be a true 64-bit operating system with 32-bit compatibility. Apple claims that 32-bit applications will run alongside 64-bit applications without conversion, emulation or virtualization. Users will be able to run both types of applications side by side in real time.
Other enhancements that come to the table include a file restore similar to what we will be seeing in Vista. There are more features to be announced, but it appears this will be a larger upgrade than compared to previous OS X versions.