The drama of King Lear is not dependent on insanity or on deception from the protagonist. Instead, this play's drama grows from Lear's flaws and the flaws of his daughters. This makes the play less contrived, in my opinion, and in this way creates a more resonant drama than what we see in Hamlet.
Yes, I think that the depth of the tragedy in this play is what differentiates it from others. When we think of other tragic heroes such as Macbeth, Hamlet, and Othello, for example, each of them are shown to be able to meet their tragic end with dignity and nobility. Even in spite of what Macbeth, Hamlet and Othello have done and their more questionable activities, we all can admire them for their bravery and courage. However, there is no such nobility in Lear. He dies a devastated man with no hope of any form of nobility. Surely the scene in which he cradles Cordelia's body in his arms is one of the most poignant in literature, as he realises his own role in her death and his destruction as a human being is complete.
The intense drama of King Lear develops from the nature and the depth of Lear's suffering. All of Shakespeare's tragic heroes suffer, but none of them experiences the depth of despair and the degree of physical, mental, and emotional destruction that Lear endures. Once a powerful sovereign and ruler of his kingdom, Lear becomes a weak old man without a home--unsheltered, blind, and abandoned--raging against the universe itself while descending into madness. And his deepest suffering has just begun.
Lear's reunion with Cordelia becomes not his emotional salvation, but the source of even greater agony. As he cradles her dead body in the play's conclusion, King Lear is broken in body, mind, and spirit--yet he continues to live, denied the release of death afforded to Shakespeare's other tragic heroes. Also unlike the other tragic heroes, Lear is not redeemed in any way, even for a moment, at the conclusion of the drama; there is no fleeting glimpse of a once-heroic trait. He has simply been destroyed, and his destruction serves no good purpose.