'The Rabbis point out that all the precepts of the Decalogue had been practised by the Patriarchs and had become the family tradition of their children' (Rabbi Joseph H. Hertz, The Pentateuch and Haftorahs, p. 403). This granted, the chiddush in Shemot 20+ is that (1) God Himself, accompanied by angels, comes down from Heaven and speaks the Ten Commandments to the children of Israel and the mixed multitude (Shemot 12:38)1 at the foot of a mountain, Mount Sinai, (2) briefly afterwards writes them on two tablets of stone, not once, but twice, and (3) these (termed 'the testimony' in Shemot 25:20,21) were commanded by God to be placed in an ark built according to His design and later called 'the ark of the testimony' (Exodus 39:35; Yehoshua 3:6) and 'the ark of the covenant of יְהוָה' (I Kings 6:19; Yirm'yahu 3:16). Though the command to keep the Shabbat had been given earlier, it was at Mount Sinai that it was for the first time linked with the creation of the world in six days and God's rest on the seventh day. Also, the fifth commandment to honor father and mother was linked to long life in the land of Israel.
1 On the term 'mixed multitude' (עֵרֶב רַב), see Shaul Bar, 'Who Were the Mixed Multitude?' Hebrew Studies, Volume 29 (2008): 27-39@JSTOR.com, where the argument is made that it refers to Egyptian mercenaries who had intermarried with the Hebrews and left armed with them.