Red had services

I almost know what my answer will be, but having very little experience with Linux (save a few commands) I am hoping that someone could maybe point me in the correct direction on services that would need to be monitored on a red hat box?

We have one "rhnsd" which is a red hat subscription service.  But save that, nothing else is monitored.  I know that people will say 'depends on what the server does' and I get that.  I also understand that Linux is at it's core similar to windows and the difference is the random, odd characters that make a command up and the fact it's a GUI.

But reading about some Linux services yields not that much.

On one hand I am thinking 'well keep it simple stupid' and only monitor what you need to monitor.  But these red hat boxes are so critical to a customer that this is their core infrastructure.  So I don't know if something is that important that it needs to be monitored or I am making work for works sake.