The Virtues of Monitoring
paperplanes.de
paperplanes.de
http://librato.com (full disclosure: I hack there) is a startup with a new kind of entry in the process-level monitoring/management space. Would love any feedback the infrastructure-minded part of the community here might have.
Rule of thumb: Think of everything you wish you had monitored after a crisis has happened (which might be "everything").
So another simple rule I learned with time is to trust/understand the defaults,plugins,knobs,metrics that come with well known monitoring systems ("why the hell should I monitor _that_?"). This way you use the experience of other people as a backup for your own.