Patch 4.0.1 is a prelude to Cataclysm. The patch contains most of the basic mechanics and ruleset changes for Cataclysm. These are mechanics changes such as the redefinition of spirit, which is now a healer-only stat. Guild reputations are in place, even though there's no way to actually generate that reputation.
This buffer period gives Blizzard developers time to adjust the servers and various back-end systems. The gears can get oiled up real well, and the developers can handle issues before they wreck the launch of Cataclysm.