What things are a *must have* when it comes to "Logging"???

Right now, I have a single "Error Log" table which logs the following when there is an error...
	- Error Type
	- Error Code
	- Source Page
	- Member ID
	- User IP
	- User Hostname
	- Created On
This seems to work just fine, but now I am wondering if there other things I should be logging, such as...

1.) When Members Log-In
2.) When Members Log-Out
3.) When People Register
4.) What Pages People Visit

And so on...

I guess I am particularly interested in if I should be doing #1 and #2. (Technically I already log #3 in my Errors Log...)

In the end, as website owner, what kinds of things do I need to log to help me manage my website, and to have an audit trail should issues arise?!