http://arstechnica.com/security/2014/02/dear-asus-router-user-youve-been-pwned-thanks-to-easily-exploited-flaw/#p3

There is always a set of standard metrics that are universally monitored (Disk Usage, Memory Usage, Load, Pings, etc). Beyond that, there are a lot of lessons that we’ve learned from operating our production systems that have helped shape the breadth of monitoring that we perform at bitly.

http://word.bitly.com/post/74839060954/ten-things-to-monitor