What do we moan about when we moan about postmodernism?

Some philosophers are very dismissive of (what they call) postmodernism while others think it is unfairly dismissed, often on the basis of ignorance. If I speak dismissively of it I don't mean the work of Derrida, Foucault, or any other philosopher. I mean a kind of ideology or cloud of tendencies that seems very widespread outside philosophy. According to this (kind of) view:

• there is no such thing as truth (or else, at least, we should not talk about truth, perhaps for some other reason, but only ever as if there are either no truths at all or else many (mine, yours, etc.)) 
• the same goes for goodness and beauty (and probably logical validity too)
• the purpose of education, therefore, is not knowledge of truth or appreciation of goodness or beauty but rather politics
• one aspect of education in politics is simply coming to accept that everything is political, that belief in truth and beauty (etc.) is naive and that to think otherwise is to fall victim to someone else's power play
• the other aspect is coming to accept that a) history, literature, and pretty much everything else show that dominant groups are (unfairly) dominant over others, and b) this is the point of studying history, literature, and pretty much everything else

Parts of this are true. Dominant groups are dominant. And this is either always or at least almost always unfair. (Murderers' being in prison doesn't strike me as unfair, but sexual and racial inequality do, for instance.) But other parts are absurd to varying degrees of obviousness. To assert as if true the claim that there is no truth strikes me as deeply problematic. Skepticism about truth (etc.) also, I think, helps right-wing extremists promote their causes. And then there is also the question of why anyone would want to study history or literature or religion or psychology or anything else if none of it is true or good and the only point of the exercise is to change your political views. Why would I want someone else to change my political views in a way that is explicitly not in the direction of truth or goodness? Why would I pay to have this done to my children? Why would governments pay for it? Why would employers want to hire people whose education had been of this kind? (I don't mean that education should be all about employability. I'm just going through possible reasons for education: to enlighten; to give students something that they want; to give students something that their parents want them to have; to give them something that the state wants them to have; to give them something that potential employers want them to have. Postmodernism as I've defined it seems to offer none of this.)

I don't know where this ideology comes from. One source seems to be scientism. Scientific claims might be true or false, the idea seems to be, but the humanities are a different kettle of fish. Here all is opinion and anything goes. Along with this is the existentialist idea that I am free, as you are too, to commit myself to anything with equal justification. It follows, of course, that racism is not actually unjust or unfair or evil or wrong, but also that we are perfectly entitled to commit ourselves to the view that it is any or all of the above. I don't know how much sense this makes, but I reject any view that denies the wrongness of racism. Nietzsche and Wittgenstein seem in some ways to be behind these ideas. Perhaps Foucault is too. But what I've read of Foucault's work is much better than this (i.e., the set/cloud of points listed above), and both Nietzsche and Wittgenstein would surely have rejected it. Variations on it, though, are very widespread, even standard, in large areas of the humanities, as far as I can tell. For instance, the religious studies course I gave up on a while ago presented a kind of relativism as a key feature of the religious studies approach to the study of religion. In The Good Story Coetzee defends belief in truth but is clearly aware that acknowledging the existence of such a thing is unfashionable, perhaps even rude:

Although, like most well brought up people nowadays, I am careful to avoid the impolite locution 'transcendent truth', I confess that privately I continue to distinguish between things that really happened in the past and things that did not really happen. (p. 74)

Coetzee is led to say this about halfway through his dialogue with Arabella Kurtz partly because she seems so deeply resistant to the idea of this kind of truth, which is not transcendent in any very mysterious way. It is simply the difference between what really happened and what didn't, between whether Don Quixote tilted at windmills or at giants. Kurtz is not alone in this resistance. It seems, as Coetzee notes, to be something people are brought up with.
 
I don't know whether anyone believes (or claims to believe) the full list of points, but something like it seems to lurk behind a great deal that is said and taught by a great many people in the humanities. And philosophers are kidding themselves if they think these ideas have all been put to bed. Of course, it might seem as though once an idea or set of ideas has been shown to be false or incoherent or otherwise implausible then there is nothing more for philosophers to do about it. But if the ideas won't die then someone needs to do more. This might be a difference between the idea of philosophy as something like a science and philosophy as therapy. Scientists prove and move on. A therapist's work, on the other hand, is never done.

I remember hearing years ago that even philosophers who are postmodernists dislike that label because it suggests something shallow and incoherent. So I'm curious about the relation between the postmodernism I'm moaning about and what we might call postmodernism proper. If the latter makes far more sense then perhaps the problem is less with bad philosophy and more with badly understood philosophy. In which case, what should we do? I'm still inclined to think that we should combat confusion wherever we find it, but that's much easier said than done. Perhaps it is simply that there are always people who talk as if influenced by philosophical works that they barely understand, and these people will move on to mangling other ideas soon enough. In short, I come and go between thinking there is a major intellectual crisis that philosophers ought to to ignore and thinking that there is really no crisis at all. But if I ever talk as if there is a crisis, or as if something called postmodernism is a very bad thing, then this is the kind of thing I'm talking (or moaning) about.