Death of the Egyptian Firstborns
Rabbi Reuven Mann, quoting Rabbi Israel Chait
Rabbi Chait said Death of Firstborns was unique: Every other plague came through an angel, i.e., a visible, physical force. For example, a great wind preceded the Plague of Locusts, and the first plague (Nile River turning into blood) involved producing some change in the water and so forth. Every plague expressed itself via a change in some visible physical phenomenon. However, Death of Firstborns was direct. There was no medium through which it expressed itself, that was apparent to the Egyptians. Until now all they saw was that G-d had some type of control over the physical forces of nature, but that He was limited, in that He had to operate through them. Thus, there was some measure of safety and possible escape. "He couldn't just will me dead with no apparent cause which I could not trace to some element in nature", the Egyptians thought.
Prior to the Death of Firstborns, G-d always operated within nature, so to speak. But in Death of Firstborns, He revealed an entirely new dimension - outside of nature - by which He can strike you down at will. Hence, the terror of that night was different than that which attended any other plague. This is my understanding and explanation of what I heard.