How were George Whitefield's sermons different from those of other ministers?

Whitefield preached historic reformed theology- very close to John Calvin, which was as unusual then as it is today. He also spoke with great passion and eloquence, it has been said that when he spoke it was 'all light and heat'. But the real source of why Whitefield's preaching was so powerful was certainly that he was a true Christian. No one could ever fault his character.