I think always, always, always add logging when there is an exception, including the message and full stack trace. Beyond that, I think it's pretty subjective to whether or not you use the logs often or not...
I often try to only add logging in critical places where what I am logging should very rarely hit, otherwise you get the problem like he mentioned of logs that grow way too big... this is why logging error cases is the ideal thing to always log (and it's great to be able to see when these error cases are actually being hit so you can inspect the problem further).
Other good things to log are if you have assertions, and your assertions fail, then log it... such as, this query should be under 10 results, if it is bigger there may be a problem, so log it. Of course, if a log statement ends up filling the logs, it is probably a hint to either put it to some sort of "debug" level, or to adjust or remove the log statement. If the logs grow too big, you will often end up ignoring them.
