Among the creators I know, one of the most common experiences they share is having been the subject of unproductive feedback. Most people, even most published authors, are just not very good at giving constructive feedback. It's inherently difficult to do this well, and too often people fall back on wholly unconstructive praise ("It's great!") or equally unconstructive decimating nitpickery.
Here is my advice for how to give truly constructive feedback on someone else's work, distilled down into the smallest number of subheads I could manage.
This is the first and most important thing anyone should do before giving feedback, and also the most difficult. It is not possible to give "unbiased" feedback; human beings are living incarnations of the biases that willed them into existence. It is possible to own up to your biases and take them into account.
You do not have to like everything you read. You should be able to set aside what you like and don't like long enough to sense if something works or doesn't work for its intended audience. If you can't do that, whether because of unfamiliarity with the material/audience in question, or because you just plain can't, politely decline the offer to give feedback on the grounds that you would be a poor source of it for this material.
I originally titled this section something like "Know the audience", but I think this is a more general and powerful way to put it. Every work is written for a distinct audience, by a distinct person, with a distinct method of delivery. Your mission, should you choose to accept it (as per above), is to meet as many of those things as you can on their turf and not yours. Do not apply a standard for something that its readership most likely will not use anyway.
Be patient. If you've read the first 100 pages of a 300-page work and something has not been resolved to your satisfaction, please remember you've only read the first third of it. Unless the omission actively prevents you from understanding what's going on, it's unfair to pin the author to the wall for such things. Most things work as a whole for a reason.
This is why some of the least constructive feedback I've encountered has been in workshops where you submit the first one or two chapters of something. Everyone wants to have everything explained immediately, when most fiction doesn't work like that anyway. A better question to ask would be, "What are the circumstances in which this element is later clarified?" From that you can derive some idea of what the reveal is like, and what its possible side effects may be, as per #4 below.
It's too easy to talk about what doesn't work. Talk about what does work, and be specific. Why is this particular character so compelling? When someone has a sense they are on the right track, that they have evoked the reaction they were looking for from at least one person, they will benefit from knowing so.
Again, for the reviewer, this can be difficult to put into words without some practice, and it can also be difficult not to couch these things in terms that are highly personal. Why you think something works must be explained as something coming from you.
The same thing goes for talking about what doesn't work, while we're at it. What doesn't work for you needs to be explained as clearly as possible. If the sum total of your reason for why something works is "I don't like it," feel free to spell out your distaste. The responsibility falls to the author to determine whether anything useful comes of that, but generally they are not responsible for nose-wrinkling in the reader unless it's chronic and globally detrimental to the work.
Some who give feedback are experienced enough to understand that certain things in a story simply don't work. How you present this to the author is paramount, because when presented undiplomatically or unconstructively, it can be easy to dismiss. "Don't do this, because XXX" is the tempting way to give advice. It's also more often than not counterproductive, because writers detest hearing the word "don't". They hear it as a dare, not as a stipulation.
The better way to position such advice is in the form of a cost/effectiveness analysis, like so: "I noticed you did this in your work. There's nothing that says you can't do that, but here's what is likely to happen if you do that. It may come at a cost that you might not want to pay elsewhere." That cost could be melodrama, meandering narrative, protracted length, a difficult-to-engage-with protagonist, or any number of things. None of them are wrong to put into a story, but all of them come with a price that the rest of the story may not be able to pay.
Making the author conscious of how their creative decisions need to remain in balance with each other and in harmony with the whole work is some of the most constructive advice I know how to give.