Warning: spoilers!
Seems to me that the central theme to the Harry Potter series, highlighted in the first and last novels, is that the deepest magic, the greatest power against evil in the world, is the sacrifice of love. The narrative metaphor in the books is that the sacrifice of love (giving one's life out of love), made explicitly by Lily Potter in the first novel and Harry Potter in the final book, nullifies the power of evil magic. The purest and most encompassing defense against the dark arts is love and self-sacrifice. Harry even gives Voldemort an opportunity to repent. He shows love toward his enemy.
This series is not about the potency but rather the impotency of magic. It's a story that shows that the greatest power in the universe is love, not secret, sought-after spells.
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