Hebrews 12:19

-20 ‘Which voice they who heard it entreated that no word more should be spoken to them; for they could not endure that which was enjoined. If even a beast touch the mountain, it shall be stoned.'

There were the fiery flames, the blackness and darkness (gnopho and zopho), the sound of a roaring tempest, the notes of an unearthly trumpet, all swirling round the top of the mountain in awesome power. And then there came the words. And the words themselves came over so fearsomely that the people who heard them pleaded that they might not hear them again. They could not endure what was said or how it was said. It was all too much for them. They could bear neither His presence nor His words. Why, His presence was so real that even a dumb beast that touched the mountain had to be stoned or shot with an arrow, because it had by that become sacred and was thus untouchable, lest it return and bring God's holiness directly among the people. They were fearfully afraid. Compare here Deuteronomy 5:23.

The old covenant was in fact good news for them. It was the gracious acceptance of them into His covenant. But what they retreated from was not the covenant but this personal and vivid experience of a holy God. They could not face Him as He was, because of what they were. They preferred to leave that sort of thing to Moses. And it would continue to be so when later Moses' face shone with the glory of God, and they pleaded that that might be covered up as well. Many of us are like that. We like to come close to God, but not too close.