Apart from a plethora of security enhancements, there are some pretty compelling reasons to take an early look at SP1. Apart from the built in firewall which is akin to the XP SP2 firewall, but better, the two features I particularly like are the Post-Setup Security Updates (PSSU) and the Security Configuration Wizard (SCW). PSSU effectively locks down the computer to stop it being hacked after installation. Note that this is only on slipstream installations. The SCW allows you to define the roles for a computer to ensure that the firewall is appropriately configured.