It is incredibly rare that I downvote on this or any other community site. I would rather just ignore the item. However, occasionally some very bad advice is given that is worthy of a downvote. Why should this adversely affect the reputation of the person who downvotes?
The motivation behind it is to put emphasis on up-voting or not voting at all. This way, down votes will carry more weight and it will also prevent users from abusing the system by down-voting excessively.
According to what Jeff/Joel discussed on the SO podcast, they wanted to find a way to discourage users from down-voting for less legitimate reasons (say a pro-Java developer down-voting everything remotely related to .NET or the like).
Edit: I have to admit, it definitely puts it into perspective to compare how many points you gain for other activities vs. the 1 point you lose for a down vote. It is effectively the smallest penalty that the Stack Overflow scoring system will permit.
Also, it may not be the best possible approach, but one has to start somewhere. Besides, the site is only a couple of months old.
Note that you only lose reputation for downvoting answers, not questions.
So how is that different than somebody voting up indiscriminately? Instead of the java developer voting down everything .NET, you have the java developer voting up everything Java. I also don't see how marking something wrong down should degrade your reputation when you are doing the community a favor. Maybe you should just get a "curmudgeon" badge.
While I see the simple votes as easy to understand, I'd rather see something like badges for answers. That way you could have things like "verified" where the voter had a choice like "I have implemented this answer and it works for me", Or "incorrect" where the choice was "I have tried this answer and it does not appear to work".
I often wonder if -2 off the person your down-voting is enough. Maybe it should be -10 to equal the amount they might get off an up-vote. I only say this as one user could have got one down-vote and one up-vote which would still equate to +8 rep points for them. (Even though their vote score would still be at zero).
Of course I understand that -10 might seem excessive, I’m just looking at a view point of balancing the rep when it comes to up-votes and down-votes.
Keeping it to -1 for the person who originally cast the down-vote but giving -10 to the person who's been given that down-vote might seem fairer to some, and make it more acceptable when losing some rep to down-vote a bad post/answer.
EDIT: I do believe that down-voting should be kept to a minimum and only used when really neccessary.
I'm not sure whether the down-voting feature was really well thought out.
On one side, Jeff praises the possibility of downvoting as a very valuable feature. On the other hand, you are discouraged from down-voting because you lose almost as much reputation as the user being downvoted.
It also seems reasonable to me. I've only downvoted once, and that was on a truly bad answer. (I've been downvoted at least three times, and I think four, and I understand why on most of them ... )
The one-point penalty is a "keep-it-honest" deal. Downvote for something BAD, but not just because you don't like it.
I'll admit to being annoyed about something in the SOverse and thinking "I'd like to downvote that". The cost associated with that helps me keep things in perspective. Nothing has yet been worth the downvote.
I should also note that when you don't have much in the way of rep to start with the cost of a downvote is proportionately far greater than for someone with a lot of rep. This favours having high-rep users as the ones who do the downvoting - which is in line with the basic idea of rep IMO.
My guess is that it is to make you think twice: Is it really worth 2 whole reputation points to mark this as bad, or should I just ignore it? It seems to work quite effectively.
Free down-votes would probably result in a habitual down-voting of mostly anything that someone doesn't agree with; a small penalty lessens that tendency.
The loss of a point on a down-vote is really a mechanism to punish newer users and prevent them from abusing the system. Let's face it; the 30K rep users here really could care less about a point, but they've gotten to that point because they've been around a while and are active in the system. They (theoretically) know better when to down-vote and when to add a comment to help the asker/answerer in question. New users, on the other hand, value every point they can get and need incentive to not just go on a down-vote binge because they didn't have their coffee.
Or at least that's what I believe.
It's a deterrent to some users who might otherwise down-vote indiscriminately. I think most people would down-vote only rarely regardless, but there are some who might want to use down-voting to penalize other users for having a different opinion, for using the wrong language, for having too much rep, etc.
The reasons for down-voting are too numerous and complex to be reliably and accurately discriminated, some down-votes are justified, others not at all. Losing reputation, and losing a single point, is a compromise, a Keep It Simple approach.
Here are various reasons for down-voting, gathered from common sens and from answers and comments above, arbitrarily ranked from those calling for more than 1 point in reputation loss to those calling for no reputation loss (or even a gain):
- retaliation: a user feels offended by some comments, or finds out who down-voted him/her and goes on a down-vote rampage
- indiscriminate down-vote: a user is in a bad mood, is upset, irritated, having a bad day, doesn't like a user, the answer...
- SPAM: question or answer is clearly written with a will to spam the site, it's helping the community to shadow these by a down vote (SPAM should be flagged, edit: flagging a post as SPAM automatically downvotes the post at no reputation loss downvote, from Catjia's comment)
- the answer contains offending wording (they can be flagged too)
- the question has been already asked, shows no effort from the user to search by him/herself
- the answer given is off-topic (similar to SPAM but in a more subtle way)
- the answer given is misleading or plain wrong
I'd sure support a no-reputation-loss when down-voting spam, and even a reputation gain when down-voting a misleading or wrong answer with a justification for the down-vote; someone who aggressively down vote for no given reason should lose more than 1 point.
But unless SE introduces Artificial Intelligence to analyze users behavior and the content of their justification for down-vote so the system can sort legitimate down-votes from those given in pure revenge, a -1 loss is a good compromise.