Sorry, race and gender people, but Marx was right, at least for our society. Class is the definitive differentiator, the definitive divider.
The difference is in the ontological categories to which the universe of associated habiti in each case are assigned, and the relative universality of these.
Sure, some people imagine that one race is morally superior to another as a matter of essence, but there is also a strong, competing discourse in society that disagrees with this position; it is a matter that is deeply contested and even in cases of strong racism, deeply conscious and intentional.
The same thing goes for gender.
For class? No such consciousness exists, and no such discourse of contestation exists. Even within the social sciences, the frame of the discussion is that of “mobility,” i.e. how to “elevate” those in the lower classes so that they achieve greater parity with the upper classes.
The stratification is taken for granted, both essentialized and rendered part of a moral narrative. Of course Vivaldi is somehow “better than” DMX, a Ferrari “better than” a Ford, wine-tasting ending in tipsiness “better than” swilling beer at a bar, etc.
Whereas there is room for cross-cultural understanding in race or the discourse on power in gender, the moral universe of class appears to be universally self-evident and all pursue a rise through the ranks as a presumed good (a presumption that is, in fact, a good one, given the concentration of necessary resources in the hands of the upper classes and their access-gatekeeping function, which relies on the same morality for adjudication of desires, claims, and merit).
In short, class is predominant becuase it’s as universal as race or gender, but we can’t see it, and never have been able to see it clearly, from the bottom to the top, so it works its black and horrible magic unconsciously, unobserved.
Nobody would presume to research how we might make black people “more white” in a drive toward “light mobility,” nor women more masculine in pursuit of “masculinity mobility,” yet “upward mobility” is routinely taken for granted as a worthy pursuit, despite the fact that the upper classes and the value of their assets and their disproportionately high access to resources are in fact socially constructed, cultural artifacts and configurations, not configurations bearing some essential higher good. And conveniently, as a matter of the structural organization of our society, they are both those most able to guide the hand of social construction, and to be the gatekeepers of the socialization processes that result and that cast one into a particular cast for life.