Theophylact of Ohrid

He shows the superiority of the New Testament in this as well. For the Old Testament, not having anything as great as the New, was something terrifying, so that according to it not even a beast could touch the mountain (Exod. 19:12), that is, any of those animals that the people had. But the New Testament, possessing all perfection, contains nothing fearful; on the contrary, everything in it is merciful. …

Commentary on Hebrews ↗

Oecumenius

"And so terrifying was the sight." It wishes to show that the New Testament also excels in relation to the Old. For indeed, even if the Old had nothing of this kind that the New has, it was to be feared in some way; however, the New, having perfection, is gentle and without terror. "that Moses said, "I am exceedingly afraid and trembling."" This was also said by the one who was about to enter into darkness.

The Pseudo-Oecumenian Catena on Hebrews ↗

Thomas Aquinas

703. – From all this he draws the conclusion, namely, that they were terrifying things, because not even the beasts were spared; hence he says, and so terrible was the sight. This indicates the difference between the New and Old Testaments, because the Old Testament was given in terror to terrify the hearts of Jews, who were prone to idolatry; but the New was given in love: 'You have not received the spirit of slavery again in fear, but you have received the spirit of the adoption of sons, whereby we cry Abba: Father' (Rom. 8:15). …

Commentary on Hebrews ↗