What is the illuminating moment in King Lear that acts as a window to explaining the meaning of the work as a whole?

One of the most impressive features of William Shakespeare’s masterful portrayal of an elderly father’s decline is the length of time it takes for Lear to realize the full extent of his daughters’ betrayal. At each step of the way, Lear rejects the implications of their behavior, somehow rationalizing their actions as less reprehensible than they are. The audience, as it sees him losing his power, his lands, and his self respect, wants to intrude into the play and intervene to save him. Lear’s massive arrogance makes it hard to like him, but his decline does elicit the audience’s sympathy. Too late, Lear must face that he has not been a good father, and that his behavior has helped create the monster those girls have become.
The illuminating moment comes during the storm, when Goneril and Regan shut him out of the castle. At first he is incredulous, fully expecting his predicament to be temporary. Once he realizes they do not care at all and he is truly on his own, his emotions take a different tack. He rails against the storm, calling down its wrath onto him. He eventually realizes that, although it is dangerous, the storm is just a composition of natural elements and its threat to him is not premeditated; the storm cannot bear him personal animosity. He addresses the storm:

I tax not you, you elements, with unkindness;
I never gave you kingdom, called you children.
You owe me no subscription. Then let fall
Your horrible pleasure. Here I stand your slave,
A poor, infirm, weak, and despised old man.

At this point, Lear acknowledges that through his failings as a king and a parent, he has partly brought this upon himself, and he learns some humility.