I'm new to Stack Exchange, so I haven't had many experiences with it yet, but something that I am finding problematic is a lack of comments to explain downvotes or recommendations to delete answers. It seems to me that requiring explanations for these types of actions would help users improve their answers, and prevent voters and reviewers from voting or reviewing against someone without valid reason. Has making explanatory comments for these actions mandatory ever been discussed?
Welcome to EL&U. Requiring comments for downvotes is probably the single most commonly made suggestion among new users across Stack Exchange, and one that has consistently been rejected since the beginnings of Stack Overflow. See e.g.
One reason comments explaining downvotes are not required is that there is plenty of help available already.
Follow that guidance, particularly with regard to prior research and good documentary support of an answer, and you're less likely to be downvoted.
If the point of downvoting were to help posters post better questions and answers, requiring downvoters to explain the thinking behind their downvotes might make sense. But in the Stack Exchange scheme, downvoting is primarily a tool for creating separation between perceived good/useful questions and answers and perceived bad/useless questions and answers.
Some downvoters do provide explanations for their downvotes that point to discrete, correctable faults in the posted question or answer. But it seems to me that most downvoters are more interested in lowering the score of what they consider to be unhelpful, low-quality, or harmful posts than in helping a poster improve a faulty post: they downvote not to instruct the poster in any practical way, but to leave the post with a worse score than it had when they first encountered it.
In this connection, it may be worth observing that downvoters (like upvoters) need not have any actual expertise, insight, or even interest in the content of the questions or answers they vote on. Post a couple of moderately popular answers to single-word request questions, and boom—you've got the 125 reputation points that qualify you to cast a downvote in response to any question or answer on the site. (The threshold for casting upvotes is even lower—just 15 rep points—suggesting that the designers of the Stack Exchange system judged that uninformed upvoting poses less of a problem for its sites than uninformed downvoting. Or perhaps they viewed malicious downvoting as a likelier systemic problem than malicious upvoting.)
Some downvotes reflect serious problems with a post. Others represents nothing more than a voter's subjective judgment that the post is "not useful." If a post receives multiple downvotes and few or no upvotes, it seems fair to interpret the negative reaction as evidence that the post has serious flaws; but if the post attracts only one downvote, or if it attracts substantially more upvotes than downvotes, I see no reason to view the downvoter's opinion as authoritative unless it is accompanied by an explicit and cogent rationale.
The main thing I want to emphasize here is that downvotes don't necessarily come from language experts and well-informed enthusiasts. Some undoubtedly do, but others come from people who don't know any more about the topic than the poster does—and perhaps know somewhat less.
For posters with high reputation point totals, unexplained downvotes are at worst a minor irritant and at best a prompt to review the post with fresh eyes and, if possible, to improve it based on one's own judgment of its possible flaws. For posters with very few accumulated reputation points, however, downvotes can be much more discouraging because they may seem to express the collective opinion of the site, as a unified cohort of language experts, that the post is deficient in some particular (but unexplained) way.
I have never accepted the Stack Exchange's thesis that downvotes are necessary to create separation between good answers and bad answers. If downvotes didn't exist at all, I think that voters would be much more inclined to create separation between better and worse answers by more regularly upvoting the better answers; truly bad answers, meanwhile, could be removed through deletion. But downvoting is fundamental to the Stack Exchange system, so newcomers to the site must either learn to deal with the baffling and discouraging phenomenon of unexplained downvotes or walk away.
My chief concern in this regard is that potentially productive site participants like user442862—who evidently wanted to know how to correct specific problems with specific posts that had received downvotes—will find EL&U so inexplicably negative and unwelcoming that they will choose the latter option. Every time we lose a thoughtful, well-intentioned EL&U participant, we diminish what this site could be.