The increase in system requirements would exclude a number of Mac systems from running Leopard, such as the 800MHz PowerBook G4 (Titanium), 800MHz PowerMac G4 (Quicksilver), 800MHz iMac G4, 800MHz iBook G4 and 800MHz eMac. All the systems affected are at least four years old.
Four years old? Shoot, those systems are archaic ... not. But seriously, you wouldn't expect a four year old system to run Vista? Or would you?