My impression is that prolific posters show up on the Top Contributors list more often than low-post-count, high-karma posters. And, of course, worst of all they don't get ranked by positive karma percentage, or by karma per post. Somebody posting a good article in Main seems to be a less common cause of showing up on the list than high output.
For that reason, I don't see it as having a positive motivational effect either. I pay loads of attention to my positive karma percentage, none at all to karma in absolute terms. If I wanted to be on the list, my best bet would be to chime in on everything no matter how low-value my opinion actually is – which appears to be a poor and occasionally frustrating use of my time. Quality, not quantity.
I pay loads of attention to my positive karma percentage
I've seen a suggestion in a comment here that you don't want your positive karma percentage to be near 100%, because it indicates you are being consumed by the hive mind.
I'm wondering what the optimal number of people on the leaderboard would be. I suspect that people who appear on the leaderboard post more often because they want to remain on it. The other advantage, is that if the leaderboard seems in reach, more people will compete to get on it.On the other hand, if too many people were added to the leaderboard, then "being on the leaderboard" would be worthless and people would only care if they had a high position.
There are currently 15 people on the leaderboard. I suspect that if there were 20 people on the leaderboard, that would increase the motivation effect, without significantly devaluing being on the leaderboard itself.
What do people think?