Generally, I think useful feedback can have different components to it.
First, what player simply didn't like - it doesn't necessarily need to be an
objectively bad design choice, but it signals that some aspect of the game might make it less successful; if no one disagrees, or even the same sentiment is shared by multiple people, dev might want to change it. And, conversely, what player did like - so that developer gets better idea of what's the main appeal of the game, and should not be given up on (especially if some others dislike that part).
Then, suggestions of fixes - how to resolve the issues the game has; possibly the most useful aspect of the feedback, as it gives dev some clear direction instead of leaving them with issues, but no way to address them. Of course, sometimes it might be hard to nail down how to improve things. I might be able to find that "this animation looks wrong", but don't know what exactly should be done to make it look properly. Still, it might be phrased like "this animation looks wrong, but I can't quite tell how to improve it" - it can spare the dev fruitless asking for details.
Finally, there can be additional ideas (related to new features, rather than improving existing ones), though it's best to avoid overly pushy tone (like "you totally should add X!", as opposed to "maybe you could add X?") - not every idea is great, and not every works well with the game concept the developer has.
Of course, another thing is the tone of the feedback - I generally try to focus on the potential ("how the game could be made better") instead of the current state ("why is the game terrible now"), and generally try not to use expletives or be harsh just for the sake of it. At the same time, people have different temperaments, and some just don't bother, or simply cannot, phrase their criticism in the motivating rather than judging way (the former is more desired for WIPs). Sooner or later, the developer will still need to learn to deal with them, and to salvage the useful information out of their feedback. The comments on
@StuffandThings85 weren't particularly nicest and phrases like "you should feel bad for creating this" are unnecessarily negative, but they did contain plenty of useful information, too.