Downvotes do not in any way prevent people from improving their posts.
Downvotes have no effect on how easily a post may be edited.
And when a post is edited, this enables voters to change their votes. (They can also change them for a short time after casting them, to help people correct mistakes.)
Furthermore, a new user starts out with 1 reputation, and it can never go to 0 or become negative. So in this way downvoting already affects new users less. Even for users who have earned reputation, a downvote only decreases reputation by 2, while an upvote increases it by 5 (on questions) or 10 (on answers).
Downvoting is fundamentally not rude.
This is similar to how it is not inherently rude to express a disagreeing opinion, in ordinary discussion, or to politely object when someone gives wrong or dangerous advice.
In fact, downvoting is an alternative to rudeness. I think at least half of the rudeness I have seen on Ask Ubuntu appears to have been the result of people not realizing that they should downvote instead of saying something non-constructive.
In addition, the ability to both upvote and downvote new posts by new users is critically important to our ability to identify questions that are well-asked and can be answered, as well as those that have serious problems and need improvement or could possibly benefit from constructive comments, or from being put on hold.
Downvoting is important for both questions and answers, but it is even a little bit more important for questions. For answers, downvoters incur a -1 decrease in reputation to discourage inappropriate downvoting. For questions, there is no such "penalty," because it is important to encourage people to be willing to downvote questions.
If downvotes appear rude or intimidating to new users, that is a problem.
That could happen because of:
- accompanying mean words. (For example, rude comments, which could be flagged and removed.)
- in situations where it might be hard for a post's author to figure out people may think is wrong with their posts, the absence of any comments to explain. However, it is much better to have no comment at all than non-constructive, rude, or repetitious comments.
- problems with the Ask Ubuntu interface. If people--especially newcomers--are having trouble using the system effectively, that's something we should work on improving.
But I think the primary reason why people feel bad when their posts are downvoted is that we are actually pathologically reluctant to downvote on this site, so the few downvotes we have are interpreted as extreme gestures. One example of mean behavior that sometimes results from not downvoting is when people threaten to downvote instead.
We actually need way more downvoting, not less.
This is a problem on some other Stack Exchange sites (though perhaps not Stack Overflow), but it is especially a problem here. Everybody on Stack Exchange is supposed to act respectfully and avoid meanness, but the Ubuntu culture is particularly serious about welcoming newcomers and accommodating people who might not have the experience to easily ask a good question (or post a good answer).
But by refraining from downvoting bad posts, we are actually hurting exactly those people, because without both upvoting and downvoting, the system cannot work as intended and bad questions cannot get the attention they need.
(There is a similar problem for answers: Bad answers and non-answers that aren't downvoted often appear above good answers and so-so answers. Also, when something that isn't an answer is posted as one, a negative score enables it to be deleted through delete votes cast by high reputation users.)
Although I don't agree with all the ideas suggested here, one of the several good things about that question is that it points out how our reluctance to downvote is harmful.
Closure can happen without downvotes, though.
Some (not all) bad questions can be closed. Downvoting does help this happen, because it helps raise awareness about how there is something wrong with them, which leads to people voting to close them.
But although a question that should be closed should often also be downvoted, this is not always the case and downvoting is in no way necessary for closure. People may downvote and vote to close for identical or similar reasons, but it's not hard to close a question that has a positive score.
It would be unusual, and probably a bad idea, for someone to downvote every question they ever flag or vote to close. For example, many questions that turn out to be duplicates of some other question nonetheless are clear, well-written, answerable, and show good research effort.
A note about meta.
Finally, please note that voting has a somewhat different meaning here on meta. For example, this meta question requests a change in the system that is not likely to be popular. To express disagreement, users may downvote this meta question. But that does not mean this meta question needs improvement or doesn't contribute positively to meta. (This is one of the reasons why votes on per-site metas don't increase or decrease anyone's reputation.)
Unfortunately, the tooltip text that appears when you hover your mouse arrow over the vote buttons on meta is exactly the same as on main. This does not fully reflect the way voting is used on meta, and is confusing.
Related: Help Center > What kind of behavior is expected of users?