This was supposed to be an addendum to a post on the structure of Sefer Vayikra. But that's still in progress so I'll just post this now.
In addition to their significance in Sefer Vayikra, parshiyot Behar/Bechukotai have special significance in terms of the larger structure of the Torah.
The end of Sefer Vayikra is an important juncture. Chronologically it seems to represent the last laws given at Sinai. Since (before the spies and the 40 years in the desert) the people were supposed to go straight from Sinai to Israel, this was originally meant to be basically the end of the Torah. The last few laws of Vayikra are a kind of concluding statement to the Torah, as well as an introduction to what should have come afterwards - the entry into and settlement of the land of Israel. Looking closely at this section, we will see aspects both of this conclusion and of this introduction.
These are all laws of social justice, but they have an interesting twist: they are phrased in such a way that they can only be applied in the land of Israel. This is an aspect we do not see in other parts of the Torah (even Devarim, despite its emphasis on the land of Israel). For example, parshat Mishpatim simply says that a slave must go free after six years - apparently a universal humanitarian law which could be applied anywhere. Whereas here at the end of Vayikra, the slave must go free (under slightly different circumstances) in order that he be able to return "to his family and to his ancestral land" (25:41). Were it not for this ancestral land, there would not seem to be a reason for him to go free. (In fact, in the absence of ancestral land to return to, this form of slavery is not practiced - see Arachin 29a.) The presumable social basis for these laws could apply anywhere, but according to their presentation here they are targeted specifically at the entry to Israel.
At the same time, these laws are introduced by saying they were commanded at Mount Sinai. Obviously these were not the only mitzvot given at Sinai, so the obvious question is why they in particular receive the "Mount Sinai" introduction. It seems that this is done in order to connect the blessings and curses in the next chapter to the Mount Sinai experience. The mitzvot which you will be rewarded for keeping and punished for ignoring are not just those of parshat Behar, but all the mitzvot given at Sinai, from parshat Yitro onwards.
The blessings and curses are a natural fit at the end of the Mount Sinai experience. After learning the nitty-gritty details of all the laws in the Torah, we step back and see the big picture of why we need to keep them - the "practical" reason of not wanting to be punished, as well as the "moral" reason of not wanting to betray the commitment we have made. At the same time, the blessings and curses are integrally related to the upcoming entry to Israel. Compare them, for example, to the very similar passage of blessings and curses in Devarim 28, right before the Jews actually did enter Israel. In both cases, the blessings and curses explain immediate consequences of this entry: either we behaving correctly and are rewarded in the land, or else disobey and are punished and exiled.
Arachin and Hekdesh
Many people find this final chapter of Vayikra puzzling. We just had a dynamite passage of blessings and curses, telling the dramatic story of the Jewish people's sin, exile, and redemption, and ending with the conclusive-sounding verse "These are the laws and statutes and instructions, which Hashem made between Him and the children of Israel, in Mount Sinai, by Moshe's hand." (26:46). There couldn't have been a better end to the book or to the whole Mount Sinai experience. But for some reason there is one last chapter, talking about the obscure laws of Temple property ("hekdesh") and vows according to a person's monetary value ("arachin"). What is the point of these weird laws, and why are they in such an awkward and anticlimactic location?
I think that beyond the technical details of these laws, there is a special symbolic meaning in their placement here, immediately after God has apparently finished giving us the Torah. It seems that these laws do not represent God's command, which has ended, but rather our response to that command. Similarly to "arachin", which are vows to give the Temple a sum equal to one's personal monetary value, here we voluntarily commit our "entire selves" to keeping the Torah whose presentation has just finished. And as with Temple property ("hekdesh") which cannot be put to any non-Temple use, having committed ourselves to a holy lifestyle, we recognize that the commitment is permanent and we can never back out.
The connection between these particular laws and the overall commitment you are making is not explicit, but if you think about it a little it is quite natural. In the previous chapter of blessings and curses, you had to listen to and absorb God's extended monologue. But religious life is not just about listening and absorbing. It is also about intuiting and creating. That is what you are called on to do in this chapter. The placement of these mitzvot is inexplicable until you put a little effort and creativity into teasing out and absorbing their symbolic meaning.
Sefer Vayikra has given us many chapters of precautions, warnings, and boundaries. But for every "shamor" there is a "zachor"; life must include initiative as well as precaution. The Mount Sinai experience began with the spontaneous commitment of "kol asher diber hashem naaseh", but after all the laws of Sefer Vayikra this attitude could easily have be forgotten. As the Israelites prepared in Sefer Bamidbar to conquer the land, they had to internalize that it is religiously appropriate to act as well as to refrain from acting. And after we modern Jews have spent two months reading Sefer Vayikra, which more than in Biblical times seems to be empty of practical significance, we must nevertheless enhance the vitality of our spiritual lives in preparation for whatever challenges await us today.