For instance, I've seen many instances of newbies making an "lol" or "wtf?" comment only to be quickly downvoted in to oblivion.
Hopefully it doesn't take much of that for them to get the point.
Also, the downvoting provides a quick and easy way for the community to express displeasure at someone violating the community's norms, without needing to write a long post explaining just why a comment consisting solely of "lol" or "wtf?" isn't appropriate.
On the othe hand, it's clear that karma is not a panacea, that it can be gamed, that it can encourage an echo chamber effect, and that it scales poorly when a site becomes as large as HN.