I don't think so. In my opinion, close-votes are the biggest problem. As I learned lately, 5 votes are enough to take a question down and then no new answers are possible, even from people that where just writing one.

I really hated it, when my questions (or questions, I wrote answers for) where closed down. You invest effort and time to write a question or answer -- but all is senseless as soon as 5 people kill the question. And also it takes forms that every question that is just a small degree beyond the "group norm" will be taken down in short time without that anybody can do something about it. 5 "Soup Nazis" (as somebody else named it) or just people that do not read carefully suffice (there is nearly no benefit, to do so -- there are some "attention" checks, but those mostly will guide you to adhere to group standards and less likely will force you to really understand the question).

I did not find out completely, but it even seems that when some moderators vote to leave questions open and 5 others vote to close, it is closed. That would be the most ridiculous thing!

BTW: I give you an upvoting, at least here, because the voting system should not only reflect opinions, but interesting views.

You would be surprised how many incorrect answers there are. I can say for certain this is happening in at least two of the top 10 tags on SO. The worst part is that some offenders don't care as long as the answer gives them a positive net reputation. Downvotes really help with this problem.

To the contrary: it has always been difficult to rank questions on Stack Exchange because there are far too few people casting downvotes on questions.

I have to disagree. Downvotes (when actually merited) serve the purpose of not only helping sort out which answers are of the best quality, but also (especially when accompanied by an appropriate comment) inform the person who tried to answer the question that his answer hasn't simply been ignored, but has actually be read and found wanting. As already noted, adding a comment telling exactly what was wrong, or how the answer could be improved is even more helpful as a rule, but a downvote can still be a useful tool.

There are a couple of limitations on that though. The first is that the downvote really is merited. The second is that if it's at all reasonable, you do add a comment explaining the down-vote. Nearly the only exception to commenting when downvoting should (IMO) be when there's already a comment pointing to the problem, so the added downvote is just to help the score reflect the answer's (lack of) quality.