My answer to this is that you monitor the things that allow you to predict and react to events that cost you money so that you can reduce that cost.
For example, I went through an effort with a new application group within my company and one of the requirements was to monitor for the change of CPU type. The supporting argument to that was "well, because we want to know". All this sort of monitoring does is cost everyone something with the only benefit being satisfying a curiosity.
On the other hand, another group looked at the past 6 months of cases, grouped by root cause, sorted them by time spent to resolve and picked the top 10 off the list and monitored for the causes of those cases. That translated to a direct cost savings across the board - happier customers, happier employees, better work flow, etc.
Underlying it I think is a subtle difference in the way to ask the "what do I monitor" question. You don't want to monitor a metric and then figure out what it means. You want to find a reoccurring issue, identify the cause, and then develop monitoring that predicts (this is important, you don't want to monitor for the failure, you want to monitor for the predicting symptoms - its like the difference between monitoring for the flow of lava in a volcanic eruption as compared to monitoring seismic activity that happens before the lava flows.... ) the occurrence of the issue.
If you are presenting data back to your non technical users, it gets trickier. If you've done your monitoring correctly (in my opinion) then you have a bunch of data that predicts a failure but isn't interesting to look at. It's like the check engine light on your car. Here's where you make decisions to monitor things that have no direct value but make for nice graphs - total memory used for instance, or total CPU. Lots of variation, you can color code so it looks interesting, people intuitively believe that small numbers are good (even though it indicates waste) so you don't need documentation. In some respects you are looking to recreate the Windows Task manager.
-Garin