Nessus focuses on the developement of security checks for recent security holes. The security checks database is updated on a daily basis, and all the newest security checks are available here and can be retrieved with the command nessus-update-plugins.
An RSS feed of all the newest security checks allows you to monitor which plugins are added and when. Traditional network security scanners tend to focus on the services listening on the network - and only on these. Now that viruses and worms are propagating thanks to flaws in mail clients or web browsers, this conception of security is getting outdated.
Nessus 2.1 is the only security scanner out there which has the ability to detect the remote flaws of the hosts on your network, but their local flaws and missing patches as well - whether they are running Windows, Mac OS X or a Unix-like system.
Get the full picture - not just half of it