The most obvious way to do this is to assess the existence of some desire according to how well its presence tends to help fulfill all your other desires. It's pretty clear, even within a subjectivist framework, that the average person is better off without a desire to jump off tall buildings.
But how about preferences which don't (at least obviously) tie in with our other desires? It might seem that such preferences are not open to criticism, so that whichever you happen to prefer is thereby best for you. This is not so. You might informedly choose one option and never regret it, yet still have been better off choosing the alternative instead. This would be so if, had you made that alternative choice, you would have been even more satisfied with your choice.
As Parfit explains it (in Reasons and Persons, p.496):
[Desire theorists] should not appeal only to the desires or preferences that I actually have. We should also appeal to the desires and preferences that I would have had, in the various alternatives that were, at different times, open to me. One of these alternatives would be best for me if it is the one in which I would have the strongest desires and preferences fulfilled. This allows us to claim that some alternative life would have been better for me, even if throughout my actual life I am glad that I chose this life rather than the alternative.
Counterfactual considerations provide subjectivists with the resources to be elitists. We can say that Rachmaninov is superior to Rammstein, because those with the capacity to appreciate both exhibit (I presume) a strong preference for the former. More precisely: for any particular individual with uncultivated tastes, they would be better off with cultivated tastes if, were they to have such tastes, their preference for them would be stronger than is their current preference for vulgarity.
So, I'm now in a position to answer my earlier question:
The subjectivist can happily grant that people may be ignorant of what would best fulfill their desires. But what about somehow who, though informed about philosophy, still fails to appreciate it? I think they are missing out on something; that thoughtful reflection fulfills a deep human need. But if they don't share that need, it is difficult to see on what basis a subjectivist could criticize them.
We may criticize them on counterfactual grounds. If only they had had an appreciation of philosophy, they would have been rewarded with all these pleasures that I am now experiencing. The extremely high quality of such pleasures is revealed by the near-universal preference of those who have experienced the alternatives. As John Stuart Mill writes (Utilitarianism, chp.2, excerpted as 'Higher and Lower Pleasures' in Glover's Utilitarianism and Its Critics, pp.62-63):
Now it is an unquestionable fact that those who are equally acquainted with, and equally capable of appreciating and enjoying, both, do give a most marked preference to the manner of existence which employs their higher faculties. Few human creatures would consent to be changed into any of the lower animals, for a promise of the fullest allowance of a beast's pleasures...
It is indisputable that the being whose capacities of enjoyment are low, has the greatest chance of having them fully satisfied; and a highly endowed being will always feel that any happiness which he can look for, as the world is constituted, is imperfect. But he can learn to bear its imperfections... because he [who is instead unconscious of the imperfections] feels not at all the good which those imperfections qualify. It is better to be a human being dissatisfied than a pig satisfied; better to be Socrates dissatisfied than a fool satisfied. And if the fool, or the pig, are of a different opinion, it is because they only know their own side of the question. The other party to the comparison knows both sides.
Update: Johnny-Dee and Clark have recently made similar points about how we clearly desire other goods than mere ('lower') pleasure. Oddly, Clark seems to consider this an objection to utilitarianism. Perhaps he had Bentham's hedonism in mind.