It's been a while since I've done a "Thoughts on ____" post. Or rather - since I've finished such a post. I have started but not finished several such posts in the meantime.
Sefer Devarim has the following structure, with the numbers referring to chapters:
1-4 - Initial speech
5-26 - Really long speech with a zillion mitzvot in it
27 - Instructions for ceremony on Mt. Eval
28 - Blessings and curses
29 - "Nitzavim" speech
30 - Teshuva etc.
31-34 "The last day" - stuff Moshe has to do before he dies
(Parshat Nitzavim consists of chapters 29 and 30, minus the beginning of chapter 29.)
I separated chapters 27, 28, 29 and 30 the way I did because chapters 27 and 29 address Israel in the plural, while 28 and 30 address Israel in the singular. Chapter 28 is the text of a covenant, to which chapter 30 is apparently an appendix. In contrast, chapters 27 and 29 are speeches given by Moshe. [I wrote about this structure for DBH way back in 2003, for some reason it does not appear in the archives.]
What should bother you about this structure is that chapters 28 and 29 have very similar content. First we have a covenant in which we are promised blessings if we obey and curses if we disobey. Immediately after this in chapter 29, we have a speech saying the exact same thing: if you disobey, you will suffer! Why the repetition?
The answer is that Chapter 28, like most promises of reward and punishment in Tanach, talks about the behavior of the entire people. If we all obey, we will collectively be rewarded, and similarly if we disobey. The chapter begins by saying: "And it will be, if you obey Hashem your God, to carefully perform all his commandments which I command you today - then Hashem your God will make you ascendant over all the peoples of the earth." An individual cannot "become ascendant" relative to a nation - only another nation can. Evidently this line - and it seems all the blessings and curses which follow it - are talking about collective reward and punishment, naturally for collective good and bad behavior.
In contrast, chapter 29 talks about the behavior of individuals: "Lest there is among you a man or woman or family or tribe, whose heart turns away today from Hashem our God" (29:17). What do these individuals do? "...And when he hears the words of this curse, he will bless himself in his heart to say 'I will have peace when I follow the arbitrariness of my heart' " (29:18). Evidently, he assumes that the curses talk about communal behavior. But he is simply an individual sinner, insignificant compared to the rest of the community. So he thinks the curses do not apply to him personally, and he can get away with his misbehavior.
Right after that, Moshe corrects this misimpression, by describing how the deviant individuals or tribe will indeed be punished.
However, Moshe then describes the results of this punishment: exile, and the destruction of the land the exiles used to live on. We are still in the speech of chapter 29, but suddenly this looks much more like a collective than an individual punishment!
I can think of two explanations for this change, one of which is my own, one which appeared in a recent issue of "Daf Kesher" (which I can't find now).
The first explanation is that we have simply gone from talking about individual people to individual tribes, and a whole tribe can be (and was on occasion) punished and exiled while the rest of the people remains unpunished.
The second explanation is that an individual sinner, if left unpunished, will influence others to do the same until the entire people is involved - שורש פורה ראש ולענה. As a result, we are obligated to punish individual sinners to prevent the misdeeds from spreading. Thus the last line of chapter 29 states that "The secrets belong to Hashem our God; but the revealed things belong to us and our children forever, that we may do all the words of this Torah." If a person sins in secret, there is nothing the rest of us can do about it. But if their sins become revealed, then our duty to punish them, and to prevent more sins in the future, comes into play.