What exactly is Moshe warning the people not to do here?
Ramban: He is warning them not to go after the Voice that they heard on Har Sinai and to ascribe to it Divinity, as if it was Hakadosh-Baruch-Hu Himself. 1
Ramban: Which in Kabbalistic terms is known as 'Kitzutz Neti'os'.
What is the significance of the reason ("Ki Lo Re'isem Kol Temihan ... ") that the Torah gives here?
Seforno: This negates the opinion of the astrologers, who believe that there nothing in the entire universe is superior to the celestial beings, and that they alone a. comprised the initial eternal existence and b. are eternal.