Not sure I've parsed your grammar correctly, but rewarding the community for participation by letting them downvote only when they reach 20 karma is a good thing.
Also, downvoting past 1 karma in general is a bad thing. It adds bias to the comment for future readers.
It really seems like downvoting in its current form is fine, and if any changes need to be made then capping downvoting at 1 karma might be a good idea.
You mean down-voting past 1 points? Why it add bias for future readers?
I also don't see why you need to reward the community. The right to down vote is not different from the right to up vote, and any user should have it. Up vote is simply "I want more of this", and down vote "I want less of this".
The same machinery used for up-votes of bad links can be applied to down-votes of good links, so you can protect the site from general stupidity.
Well, when you downvote, you're really saying "The community should have less of this", not "I want less of this". Karma is global, not different depending on which user looks at it.
Therefore, if the community has deemed a comment as stupid (karma < 1), any future readers will read the comment through tinted glasses. But wait a minute, the community really hasn't said that. It was only one or two users that did.
Let's reexamine the purpose of karma on a comment. The purpose is not simply to punish the poster of a comment if it gets downvoted. It's to move the noise down to the bottom of the thread, where it can be ignored.
So I propose an alternative: Let silly comments stay at the bottom with one karma point, and encourage users to upvote more interesting comments higher. This way, users won't be biased against any comments (They won't know if a comment has been downvoted because it might simply be that no one has upvoted it) and the noise can still stay at the bottom.
The problem is that each vote change the number of points, so if you happen to be the first down-voter, it will become zero. But if another user up-voted the same item just before you, then your vote change nothing.
I seems that reddit solved this problem by not showing the number of points for new messages.
There could be a "new" status, marked in a special way, which explain why you vote it not showing. After enough users voted, and using the editor vote to change the weight of other votes, move the item to voted status, and show its point count.
