What is special about the words “who keeps covenant and steadfast love with those who love him and keep his commandments”?

As with the phrase great and awesome God, these words are borrowed from Moses instruction in Deuteronomy 7. Moses had pressed upon the people of Israel that the Lord had chosen them for his treasured possession (Deuteronomy 7:6). Then Moses drew out God’s identity: the Lord your God is God, the faithful God who keeps covenant and steadfast love with those who love him and keep his commandments, to a thousand generations (Deuteronomy 7:9). By adding these words to his earlier quote from Deuteronomy 7, Nehemiah is deliberately reminding the God to whom he prays of his own self-revelation through Moses long ago. That self-revelation provides ground for his coming petitions. The quote also guides the thinking of those who listen in on Nehemiah’s prayer.