The GR”A explains the fundamental difference between Sefer Devarim and the rest of the Torah is the level of prophecy. He says that, for the first four books, God spoke directly through the throat of Moshe Rabbeinu in real time. Sefer Devarim was prophecy on the level of the other prophets who usually had their prophecies at night while sleeping and then told them over to the people the next day while awake.
