Um. From experience (I'm very reluctant to give examples, for obvious reasons): Comments that don't take much thought to come up with seem to net around as much karma as moderately thoughtful ideas, and can be produced in much less time and effort. Of course excellent insights shoot through the roof, but someone who can't produce many of them in a lifetime can get much more karma pointing out the obvious and making silly jokes than racking their brain. Also, basic ideas in domains LW knows little about gain a lot of karma, which is defensible as comparative advantage, but so do well-phrased restatements of basic ideas on LW.
I'm not claiming that the karma system successfully elicits insight-optimizing behaviour from LW participants (so, in particular, I don't think anything I've said is inconsistent with saying that some people can get more karma from pointing out the obvious than from racking their brain). Only that there's some correlation between karma and quality and that an intelligent, knowledgeable, wise person will (for any given level of effort and matchedness between their ideas and LW's collective prejudices and interests) tend to do better than someone less intell...
Precommitted to publishing this in Discussion to fight publication bias. It looks like intelligence (as measured by IQ, SAT scores, etc.) isn't meaningfully related to how much one posts to LW. Probably in the ideal case, they would be related and higher-IQ people would post more, but that doesn't appear to be going on either.
How well-educated you are doesn't seem to be much related to participation either. I'm not controlling for hours spent on LW for any of this, though.
Script output:
Script source here.