Chazal bring numerous differences: Hashem wrote the first Luchot, while Moshe wrote the second ones; the 1st set was miraculously readable from both sides, while the 2nd was only readable from the front; the 1st set was made of sapphire, while the second one was just made of plain rock; the first set had the entire Torah written on it, but the 2nd only the Aseret Hadibrot. The word "tov" is only mentioned in the 2nd set of Luchot (in the Mitzva to honor one's parents). The Gemara (Bava Kama 55) says the reason is that the first Luchot were destined to be broken & Hashem did not want the "tov," the good destined for the Jewish people, to be "broken" with the Luchot. In the 1st Luchot it says, "Remember the Shabbat day," in the second it says, "Keep the Shabbat day." The Pesikta explains that the word "keep" is used to teach the Jewish people that only through "keeping" the Shabbat would they succeed in "keeping" the second Luchot from being lost like the first Luchot.