As a game developer myself: game design is much harder than it seems on the outside. There are many tiers of criticism and validity thereof. From lowest to highest:
- The "this game sucks design". This is obviously neither helpful not insightful.
- The "I don't like this element". This at the very least starts to home in onto what you find objectionable, but still leaves out 99% of the intellectual work of deciding if it really needs to change, how it affects the overal design, and what to replace it with.
- The "this element detracts from the overall design, for this reason". Assuming the reason is valid (a big if!) this starts to resemble actual, helpful criticism.
- The "this element would be better replaced with this element, for these reasons". It's very rare that a criticism gets this detailed and still gets it right, because the more specific you get in your prediction, the easiest it is to get it wrong (this is a basic property of predictions). But assuming the reasoning is solid, this is the most helpful of them all.
Not all criticism needs to be of the fourth kind to be valid; even something as vague as "this element feels / doesn't feel fun" can be helpful, cumulative feedback for a developer that is still exploring the potential design space of their game, to lean on the fun and move away from the unfun. What I find most frustrating is mistaking the helpfulness of this feedback with the implication that one has a better grasp on game design than the actual game designer. "Remove this" is as helpful, informed and aware of cascading consequences as taking the bottom card from a card castle.
Be as specific as possible. The design space of "don't do A", like most negatives, is infinitely larger than "do A"; you don't get designer points for "narrowing" the solution to the former. Everything holds up on paper; until you don't actually roll up your sleeves and start turning that paper into a game, you won't realize the multitude of ways in which your design breaks down and needs to be replaced with something else.
With time and experience, a good game designer can map and simulate in their heads whole gameplay systems without implementing them, and guess where they will not work, or break down. This same skill can be used to mentally simulate suggested changes in the design, which can come across to others as designers "not even bothering to try it to see if it works". This is 100% a skill you cultivate, and while playing a lot of games over the years also helps a lot, much of it has to be experienced first-hand by getting into the nitty-gritty of games as entities with hundreds of interlocking parts. This is why game designers are much more likely to listen to other game designers; they are more likely to come up with solutions that are complete and actually work, because they're used to making these mental simulations as their day-to-day work.
This got way too long and ranty so to sum it up:
- The most valuable feedback, by far, is that which comes with a potential solution that could actually work.
- You don't need to be a designer to provide feedback, but experience makes it much more likely that you will come up with a solution that actually works.