Sometimes I look at a story & think what rubbish, so refrain from commenting rather than crush someone's fragile hopes (after all sending your work 'out there' can be like taking off your clothes in public). Then I will see that others have added valuable feedback. What is the best/most useful way to approach reviewing someone else's work?
I'm going to put myself out on a limb here because I've been watching of lot of the shared work and the comments that go with it. Looking at it overall ...
It doesn't matter how constructive our criticism is, it has often not been well received or the item we've criticised has been defended or removed. Anyone who puts there work up here needs to understand that not everyone will see their work the way they want us to see it. In other words we need to be prepared for comments that aren't always what we want to hear.
Sometimes people don't comment because they don't enjoy the genre. I am one of those. I don't like any violence and I don't enjoy thrillers or fantasy; so I will never comment on them, even if the work is good. More often than not I don't even read them.
Honesty is a valuable commodity when commenting on other peopel's work but it should be considerate honesty. You could try saying what you like about it. If you say what you don't like about it, that is also a fair comment. The important thing is how you say it. Don't rubbish anyone's work. Look for the positives, and negatives (if you think it important to mention and not that it is just not what you like to read).
If you can't be contructive about the negatives, just focus on the positives. Most authors are looking for comments that will confirm their decision to write something and will appreciate it when someone else indicates that they are getting it right. Equally, if something does not work, give a good reason why you think it does not work, rather than just say that it didn't work for you.
As part of my 'day job' I coach people and the best way I find to get feedback across without destroying someones confidence is the feedback sandwich.
Point out something positive
Give them the negative (which should be constructive)
End on a high by giving them something positive and encouragement
I think with writing it's also important to remember the difference between personal likes/ dislikes and something that has the potential to stop the writer getting published.