Then they said to Moses, "Speak to us yourself and we will listen; but let not God speak to us, or we will die."I've always wondered at the source of the Israelites' fear of God's voice at this point. I see two main possibilities for that answer. The first is that the Israelites feared God's power, having had many demonstrations of what He could do should they displease him. He already did terrible harm to the Egyptians, and wiped out the Egyptian army for good measure, so they know full well what could happen to them if He should decide this was all a bad idea.
The second possibility is that they feared God's holiness. For this, I take my example from Isaiah, when he sees God. He realizes that he is unclean, and unworthy to be anywhere near God, and fully expects to die for the simple act of being within sight of Him. Here God had just spoken directly to the entire nation, giving them their first rules, and they know full well that they're already in violation. So perhaps they are reacting out of fear that they are already judged and found wanting, and nothing they can do will change that, so they must maintain some type of separation between themselves and God, for their own safety.
I personally think the first option is more likely, but I wish the second were true. Either way, it is clear that something has changed between then and today. In modern Christianity, people long to hear from God, to have that booming voice tell them what they should be doing, how they should be living their lives. I can't say that I've ever heard that voice, booming or otherwise, more than once in my life. Yet in that yearning, we seem to have lost the reverence for God's holiness, the awareness that He holds the power of creation and destruction of anything should he find it unworthy (which by implicit definition is everything). How would we lead our lives differently if we kept awareness of how powerful God truly is, and how unworthy we are to even be close enough for hearing His voice to be possible? By grace he welcomes us in, and we tend to take that privilege for granted.