We live in a society in which it is very easy to find people complaining; and, what is more, people take themselves as being entitled to do so. But it's also fairly easy to recognize that not all complaints are reasonable complaints.
In certain kinds of legal systems there is a concept known as standing or locus standi: in order to bring a lawsuit, you need to have standing. The exact doctrine of standing varies somewhat from legal system to legal system, and can also vary according to the case, but there are often common themes. A fairly basic kind of standing has three elements, which can be roughly stated in the following way:
(1) Injury in fact: there is a genuine (legal) harm, one that is specific and actual.
(2) Causal relation: the harm is genuinely an effect of the specific behavior being challenged.
(3) Redressability: the legal action can reasonably be regarded as able to provide appropriate relief for the harm.
There have been attempts here and there in recent decades to argue that doctrines of standing are out of date, but I actually think that they are just specific forms of a more general set of principles concerning reasonable complaint and protest. That is to say, I think legal standing, and the above three elements especially, can be justified as preventing courts from wasting time on complaints that by their nature are unreasonable; precise details might need, and perhaps often do need, adjustment for this or that specifically legal reason, but the general structure is quite necessary. (There are other reasons for a doctrine of standing, including its value in limiting government overreach, but they are less relevant for my purposes here.)
Thus, I think one can generalize the notion of locus standi to cover all complaining of any kind; complaint without locus standi is unreasonable complaint. And, as with the above basic notion of legal standing, three principles seem to stand out as quite necessary for rational behavior. They can be formulated in a positive or a negative way.
(1) Reasonable complaint is about what is genuinely harmful. / Complaints about things that do not actually harm are unreasonable.
(2) Reasonable complaint depends on the fact that a harm is genuinely an effect of that about which one is complaining. / Complaints about things that do not actually cause the harm are unreasonable.
(3) Reasonable complaint is connected to solution of problems. / Complaints about things that cannot be remedied, or that are divorced from any remedy, are unreasonable.
There might be some pushback on the third, but it is in fact true that people often criticize complaints about the inevitable or the incorrigible, and it is difficult to give any account of the practical rationality of complaining about what one cannot change. As to complaints in cases where the problem could be solved but in which the complaint has nothing to do with actually solving the problem, people often criticize this, too, and allowing it creates a number of problems in practice (e.g., people interfering with actual solutions by their incessant complaining, or exasperating people who are actually looking for real solutions).
Thus one can assess the reasonableness of complaints by looking to see whether they have locus standi: Is the thing complained about a real harm, and why? Is the thing complained about really the cause of the harm, and why? Is the complaint contributing to a real practical solution to the problem? (It hardly needs to be said that complaining on the internet often fails these tests. Some of that which does could, perhaps, be taken as mere venting, which is the kind of action that is not reasonable in itself, but at least arguably could be reasonable in a particular context. And not all unreasonable action is morally culpable, of course; some of it is just itself a harmless waste of time. But we all know that it would be difficult to stretch considerations like these very far.)
(1) This account is about complaint. Not all critique or criticism is complaint; there can be a considerable difference, in fact, between criticizing a position and complaining about it. However, I think there's reason to hold that the above principles of reasonable complaint are themselves specific forms of even more general principles of reasonable criticism. Even so, it is a matter for inquiry whether there are kinds of criticism sufficiently different from complaint to operate under very different kinds of principles.
(2) The above principles are almost certainly not exhaustive. There are many things to consider in reasonable action -- honesty, moderation, fortitude, appropriateness to context, justice -- and while some of the basic principles of these might reduce to the above three, I see no reason to think that they all do.
(3) As noted above, unreasonable complaint is not itself something that it is necessarily reasonable to complain about. Complaint may be unreasonable but harmless; or it may be unreasonable, but any harms associated with it are, for all we know, not associated with it at all; or it may be pointless to complain about. But this is going to be something that can only be determined on a case-by-case basis. And, of course, merely because a case of unreasonable complaining is not harmful doesn't make it any more reasonable.