* ZFS pool errors. Motivator: one of my HDDs failed and it took me a few days to notice. The pool (raidz1) kept chugging along of course.
* HDD and SSD SMART errors
* High HDD and SSD temperatures
* ZFS pool utilization
* High CPU temperature. Motivator: one of my case fans failed and it took a while for me to notice.
* High GPU temperatures. Motivator: I have two GPUs in my tower, one of which I don't really monitor (used for transcoding).
* High (sustained) CPU usage. I track this at the server level, rather than for individual VMs.