Throughout much of recorded human history there were almost no complaints about inequality even though inequality was much worse then.

There were quite a lot — but they were suppressed with a totality of violence that many modern readers might erroneously think had been invented as recently as Hitler, Stalin, or Mao.

Look up the Albigensian Crusade sometime, or the Savoyard massacre of the Waldensians — complete with mass rapes and mutilations. Many medieval "heresies" seem to have been mass movements with explicit doctrines rejecting both ecclesiastical and feudal hierarchy, and asserting religious and social equality among all believers.

One problem is that the victorious hierarchs tend to systematically burn the books of the heretics, so records of them are poor, and progressively worse the further back in history we look. However, we should expect that the above cases are not unusual, that (until the Enlightenment and mass literacy) one social era was much like another; and thus that there have been similar movements — and similar massacres and suppressions — throughout human history.

For that matter: http://en.wikipedia.org/wiki/List_of_peasant_revolts