What do you monitor on devices?

I know this varies by situation a dozen other things, but i was wondering what are some things you monitor on switches, routers, UPS, WLCs, etc.

I know there are more useful items to monitor aside from just up/down status.

I've been tasked with completely redesigning our zabbix monitoring system. In many cases the default templates have been used all all that comes with them. I was wanting to slim this down to what we absolutely needed.

I appreciate any input, thank you.