You might want to look at the list of tests that are available for Big Brother, Big Sister, Nagios, MRTG or other network monitoring tools, and see what's on their list that might be useful, that you haven't thought of yet.

Personally, I look at two types of monitoring -- alerts for when something's having problems or going to have problems soon (eg, a disk is almost full, a webserver's taking too long to respond), and historical records, so I can try to spot trends and/or capacity planning. (eg, when I worked for a university -- is this measurement a true problem, or just part of a normal cycle (like usage spikes near end/beginning of semesters))

Although many people monitor to send alerts, I found the second to be more valuable -- You can trace back when memory/load/disk usage started going up, but before it hit alert levels, and find what changes were made shortly before that might be causing the problems. You can notice abnormal behaviour (the load goes up every Tuesday morning from 3am-9:30am? Maybe it's a cron job that needs to be moved forward so it completes before the workday starts), etc.