Not always in reality, but in theory I think that if a post is 'great' (I know that's not easy to define) the author deserves 'quite some' (again it's not easy to name an exact number) reward. :)

I'd rather get 10 fractions of a penny from 10 different people than 100 fractions of a penny from one person.

You seem pretty well informed, is there any way to get a page view counter integrated that might be able to roughly estimate the number of people who spent more than 20 seconds reading a post?

...for example 'diminishing returns', which wouldn't forbid to upvote the same users again and again but made it less beneficial.

That sounds promising. Something like one vote per person per week at 100% and then if you vote for that same person again within the same week, your max vote gets cut by 10% each time?

I agree with you that the rewards for many short posts from 'popular' authors (often witnesses) are way to high, considering the information they offer and their content quality.

You've guessed correctly.

(at)cheetah consistently gets over $20 per post for basically automatic spam (log files).