My current network is a work in progress.
Im pretty much inline with what everyone else is saying. We only monitor uplinks and core interfaces. I monitor both physical and logical interfaces such as Port-channel and VLAN interfaces. We also have centralized syslog and Trap collection in multiple locations. Netflow is in place as well in all the key areas of routing.
Next step for me is to better monitor my routing tables and BGP peers and also fine tune our alerts.
Im also still in the phase of getting everyone on our team familiar with our SW NMS and netflow appliance. My end goal is for the whole team to use SW as their first response to any issue instead of SSHing into the effected device. Slow and steady!