Just about everyone knows that authors generally love comments on their work, even if some of them are afraid of trying to reply to them and don't know what to say (like me). Comments tell the author what people liked, what they didn't like, what they can improve on and very often can just be an excellent source of encouragement.
However, as I read through the comment sections of stories, I notice that many people have a tendency to do things that can be very discouraging to the one writing, and overall can just be a downer for the day. Noooooot everyone writing comments cares about whether or not they hurt the author's feelings, unfortunately, but I wrote this three-step guide for those who do.
Errr... well... "three steps." I think the third is the shortest.
1. Don't comment if you don't need to.
That's a really weird one to start off on, right? I just finished talking about how great comments are! Well, what I mean is this: if your only purpose is to make a comment about why you aren't going to fave/vote/like/read it, then you probably don't need to comment unless you include some constructive, correctable reason (i.e. a lot of bad grammar). Otherwise, the only thing you really do is discourage the author. As an example, Someone recently posted this on a story here on FimFiction:
"Personally I'm not going to track this because it lacks one prime feature that I require to read stories and that is humans. While I will thumb you up because this sound like an interesting story, I will not continue reading it, sadly."
One might as well say to the author, "I'm sorry, but your writing wasn't interesting enough to engage me outside my specific interests." In my opinion, the commenter would have been better off just saying, "The chapter was interesting and well-done. You get my upvote." By all means, a person can read only stories with humans if he or she wants to, and it's great the person decided to give an upvote, but commenting on how he's not going to read it despite how interesting it is because it doesn't include humans is... kind of pointless, especially on a pony-centric site where "human in Equestria" stories are the minority.
Ask yourself what purpose you honestly hope to achieve by that kind of comment: Do you want authors to redo their stories with -insert personal taste here-? Do you want them to only write stories with that preference? Do you want them to take the story down because it doesn't include that preference? No? Then keep your comment relevant to why you are giving them an vote or a fave (if you do) and leave it at that. Whether or not you continue reading a story is a personal choice. Saying that you're giving them a vote despite not reading it or not continuing to read it doesn't make authors feel good, it makes them feel dismissed and patronized, and still makes them question whether they did a good job.
2. Giving "criticism" isn't enough. It needs to be constructive.
Giving a bunch of reasons you don't like a story doesn't count as "constructive criticism." The comments aren't there to serve as a personal gripe sheet, and far too often I see people commenting over things that amount to personal taste. "I don't like this ship." "You shouldn't write that as feet, you should use hooves." "I don't like the way you portrayed X character, and I don't care what the show did; I think that was a bad decision too." "The color scheme on that character is too cliche."
You all know the saying, "Opinions are like butts: everyone has them." That's not a bad thing either! We'd literally be lost without opinions and it's good to form opinions. The person who doesn't like a certain ship or a certain presentation of a character may have a list of justifiable reasons. The person who thinks one should never use "feet" in reference to equines... definitely has an opinion, even if I heartily disagree with it (I mean, really, I'd like to have a serious anatomy discussion with anyone who thinks horses don't have feet - it's like saying we don't have toes because we only see our toenails). You get the idea. We all have them. Our reasons for them are a side-point.
When people make comments on those things, it tells me that they aren't really trying to read the story for what it is. Instead, they're imposing their own ideas of what the story "should be" and judging it off of that. The merit of a story doesn't reside in how well it fits in any pre-conceived expectations we have, but in how well it communicates its own world and setting. It's not always bad to state such opinions, but even if you're going to measure a story by them, they don't really have any place in the comments because your opinions aren't the ones the story was written out of. If anything, they should be offered as a helpful alternative perspective to the author, and if it's a major revolving point of the story (i.e. "I don't like this ship") remember the story won't be rewritten just because you don't like the ship, so it's irrelevant to mention in the first place.
So, make sure your criticisms are actually constructive and not just a list of things that didn't fit your tastes. For example, if the author was trying to present Twilight Sparkle in a certain light, and you don't feel the author pulled that off very well, let them know and give tips on how to better achieve it if you can. If you feel a scene was weak and could have used more emotion or description, mention it. If a certain passage or the story structure was confusing, bring it up! And again, always try to give perspectives that are correctable. Don't just say "this was horrible" and leave the author at a loss as to why or how to fix it. If you're going to take time to comment, take enough time to give the author a solid direction to work from.
3. Being nasty in criticism achieves nothing.
I know, I know, I know. The people who write nasty comments aren't going to care that some wingbag says they shouldn't. Still, I'm going to go ahead and put it out there. If you can say something in a disparaging way, you can probably find somehow to say it that isn't aimed at making the author feel like the worst person on the planet. A badly written fic doesn't qualify it for open ridicule. Ridicule isn't constructive nor is it necessarily even criticism. It's just mean, and saying it might "get through to the author better" isn't a reason; it's an excuse that assumes the author won't respond to anything less than a scathing comment. I've seen a bunch of "bady written" fics where the author responded very positively to good, constructive criticism.
Encourage people to write more. Don't aim to make them feel like they should drop off the face of the earth because they aren't the best with their grammar or a certain aspect of their writing, and don't try to make them cower in a corner. The only time I'd feel a harsher criticism warranted was if I felt their story was the result of laziness, and I won't assume laziness unless I see it in the author's attitude. It doesn't make you look better to make them look worse. Intelligence doesn't smother stupidity: it nurtures good sense.