Why does downvoting an answer costs 2 reputation points while doing the same on a question not?
In short, questions aren't what we want. We want answers. Great answers actually.
To get great answers, we need great questions. The bad questions should die a horrible death. And they can only die that death if people downvote them. And people don't like downvoting when it costs them
So, why don't the same rules apply to answers? Because we can't have people manically downvoting all the possibly great answers for the fun of it. Judging the quality of an answer should be done carefully, the reputation cost incentivises that.
A much better explanation of this is also available in the blog post Optimizing For Pearls, Not Sand.
Jeff initially proposed this in May 2011: Should downvotes on questions be “free”?
Since the community agreed, the proposal was implemented soon thereafter. The rationale behind it is that a question & answer site needs good questions to survive—or at least be more efficient at telling the good from the bad questions—and that removing the cost of downvoting questions would get users to vote more.
Perhaps we should institute a new policy: every time you forget to vote a great question up, or a bad question down — a kitten gets it!
The follow-up blog post is here. It talks about optimizing For pearls, not sand and the value of good questions, which basically comes down to incentivizing (I hate that word) you to downvote the bad stuff.
That’s why we’re determined to keep question quality high, even at the cost of refusing a little sand. It’s true that you can’t have Q&A without questions, but having the wrong sorts of questions is far more dangerous. The fastest way to kill any Q&A site is to flood it with low-quality questions.