The difference between Sefer Devarim and the rest of Chumash
A number of the mefarshim (Gra, R' Y Kamenetsky, the Ramban seems to say this, and I heard this from RHS as well) make the following distinction between Sefer Devarim and the rest of Chumash. The first 4 sefarim was Hashem talking through Moshe's mouth. Every word was literally dictated by Hashem. Devarim however, is Moshe expressing Hashem's ideas in his own words. A regular Navi gets a vision from Hashem which expresses a certain idea which the Navi then says over in his words. Sefer Devarim is the Nevua of Moshe. Moshe received the ideas from Hashem more clearly then any other Navi, but he expressed those ideas in his own words. This explains why the style of Sefer Devarim is different from the previous 4 sefarim. This also explains why certain rules of Torah SheBaal Peh are different in Devarim, for example everyone agrees that in Devarim we darshen semuchim.