A particular example of a recent top-voted comment comes to mind (I'm not going to identify it because my purpose is not to name and shame), which was full of perfectly legitimate and constructive criticisms, but the tone of the comment was, well, mocking.
And what's the point of that? Making legitimate criticisms and dressing them in derisive language is a great way to raise an army of contempt against someone, but why?
If you want the target of your criticism to take the criticism into account, you need to express it civilly. You don't need to find positive things to say, or even claim that you think there's a kernel of good idea to be found (although genuine compliments are great too, of course). All you have to do is be polite and respectful. Imagine that the creator of what you're criticizing has just shown it to you excitedly. Read your comment, and decide if what you're writing is something that an acceptable member of society would say to that person face to face. Not the content, but the actual words.
And if you don't want the target of the criticism to listen to you? Then maybe you should just keep your thoughts to yourself.
I can't remove the comment now, but I can consider making my criticism more constructive and positive. With the email, my concerns were legitimate, but I could have just expressed what those concerns were, not satirized the original email.
There's no denying that writing a smackdown is fun. We just have to remember that it's not harmless fun.