If everyone had identical criteria for voting, we would see all postings having either large positive karma, karma near zero, or large negative karma. The more alike people are in their judgements, the less information the total score provides.

If you only can give 1 plus vote, 1 negative vote, or no vote at all, that seems to follow. If you rather could give, say 1-5 positive or negative Karma, we would see a greater variety of scores.

Also, note that many posts and especially comments have very few votes. This means that the votes actually cast will often not be typical of the whole population of possible voters in a system where people's votes vary considerably. In a system where people's votes are more alike, this obviously happens less frequently.

Yes! One click! A more complicated system would not be too complicated to use, but too complicated to be worth using. On Ebay, I'm happy to give feedback as positive/neutral/negative plus a few words of boilerplate, but I never use their 5-star scales for quality of packaging, promptness of delivery, etc. How do I rate a cardboard box out of 5?

I agree that one shouldn't have to rate, e.g. comments on say five different criteria. The system could be be somewhat more complex to comprehend, but you're right that it shouldn't be significantly more complex to use.

I think one obvious improvement is, though, to separate the posts into different categories which are to be assessed on different criteria. You could have one "objective information/literature review" section, one "opinion piece/discussion" section, one "meetup" section, and possibly a few more. In each section, you'd be rated on different criteria. That way, original pieces wouldn't be downvoted because they're not literature reviews, which seems to be Gunnar's (justifiable) complaint.

This system would be superior to the present, and no more complicated. I think further improvements are also possible, but those should be separately discussed.