From a world-historical perspective, not much at all. As every time traveler knows, the only way to avoid paradox is to leave things exactly as they were to be, and as every historical materialist (in Walter Benjamin's sense) knows, history only became history because the alternatives to it were already corpses in their own times.
Yet in the tunneling symbolic spaces between the real and the imaginary that can, with increasing anachronism, be called a novel, everything happens, and the provisional name of that everything is Douglas Lain's Billy Moon. This Lacanian diagram of a book is designed to confuse. Yet whereas obscurity in theory is the symptom of the will to power, an attempt to obtain and secure mastery through intellectual force and law, in literature it bespeaks an attempt to be realistic, not by demanding the impossible, but simply by allowing it to be.
Philosophers have critiqued ideology in various ways, but the point is to stop living it. This meta-critique is implicit at several points in the narrative, but nowhere more starkly than in one of the "probabilities" (i.e., unknowable near-endings):
Gerrard was dreaming, but the dream was not his own. Gerrard had thought that knowing that it was a dream, that lucid thought, would be enough, but the dream had a structure, and there were dream police.
May 1968 exerts a pull on the radical imagination because it was the first near-insurrection in conditions of modern capitalist affluence. One hopes for the sake of human survival that it proves not to be the last, or that it does only because future such events lose the prefix "near-". It is a moment in the structure of the dream.
As those who stand in solidarity with that moment find in reflecting upon it, to escape alienation it is not enough simply to live one's dreams, for those dreams may or may not be our own. Or rather, our daily reality is already a dream, an enervating play of abstractions. What could be more realistic than to wear a hat, have a job, bring home the bacon, and yet all but the hat are already fantastical. (And if the hat is the commodified product of exploited labor from a distant land, fashioned to evoke some television character who is fashioned to evoke some film character who was fashioned to evoke the actor portraying the character who has himself been fashioned to evoke a crude archetype, even the hat is not simply a hat.) To face reality as it simply is, rather than as we imagine it to be, would be revolutionary in its significance, and as simple as walking to the North Pole.