Here is my perspective, as more of a longtime user of this site.
In general, explanations for downvotes are not required, and so shouldn't be expected.
Downvotes are ... something that different people use differently. There are a few clear rules (don't downvote just for reasons not related to a post's content) but in general, I don't think it's useful to talk about "productive" and "unproductive" uses of downvotes in contexts like this.
This question does not show any research effort; it is unclear or not useful
is the downvote tooltip for questions. Many people use downvotes to convey this message. It's not necessary to write an additional comment to explain what the downvote means; this is, more or less, what it means. I think it's fairly clear that the linked question does, in fact, show little if any research effort. It's fairly clear, but could benefit from some edits to clarify the source of confusion. People will have very different ideas about if it is "useful".
So overall, it's not too surprising to me that it was downvoted. As long as Stack Exchange works the way it does, some downvotes on questions like this are inevitable, so I would recommend not worrying about them.
People nearly always have the opportunity to redeem themselves. Posts can be edited, and votes can be retracted. Both of these actions are relatively rare. Even if the specific person who downvoted doesn't retract their vote, other users' votes on a question can easily improve its score.
People have different opinions about whether new users should be given more "latitude". I tend to feel like it's somewhat irrelevant. It's possible that being more lenient on new users will allow them to slowly adjust to the site. It's also possible that it will just extend the period of time when they post low-quality questions, because they don't get quick feedback. That's an empirical question that I don't know the answer to, and I'm pessimistic about my chances for finding it out.
I'm sure the rate of downvoting affects how "friendly" people perceive this site to be, but I don't know the practical effects of that either, and I'd imagine that there is a wide variance in perceptions of "friendliness" from person to person anyway. Some people are quick to see aggression, while other people are thick-skinned and don't care if they seem to have caused offense.
Actually, I'm somewhat skeptical at this point of the whole concept of voting on questions, and the voting + reputation system in general. Do downvotes really cause users to change their behavior for the better? I have no idea. Maybe they more often cause users to behave worse. But that also isn't clear to me. A single downvote on a question has very little obvious effect on anything: at most, it removes two reputation points, a pretty trivial amount (even for a new user). Note that for new users, downvotes may not even affect the reputation score at all, since you can't go lower than one. That's another reason why I think it isn't worth worrying about downvotes on questions like these.
Nowadays, I mainly vote on answers, since that has the clear effect of contributing to the ranking of the answers on a page (and I also vote up questions that interest me, just because it feels natural I guess).
I have downvoted questions without leaving comments in the past. There were various motivations for this; sometimes the following considerations felt relevant:
anonymity. If the questioner seems antagonistic or rude and I don't want to get into an argument, I might value the anonymity of a downvote
Not being able to think of anything useful and nice to say. It takes effort to write a good comment. Sometimes, I feel like it isn't worth the effort: e.g. if a question seems likely to be deleted soon, or if it seems so low-quality that I think it's unlikely the user who asked it will become a helpful member of the community.
An expansion on that: in my opinion, too many users on this leave patronizing, mocking, or superior-sounding comments on bad questions. (E.g. beneath a question about some grammar error, someone "cleverly" imitates the error in some comment saying the question doesn't belong on this site). I definitely think I have been guilty of doing this, but I want to try to avoid it. I feel like I'm being more rude if I downvote and leave a comment that can come across as mocking or patronizing than if I just downvote without any comment.
