Why will Yaakov's offspring be specifically like "the dust of the earth"?
Targum Onkelos and Targum Yonasan: There is nothing that is more numerous than the particles of dust. 1
Ha'amek Davar: Even when you will be at your lowest in exile, you will flourish in all directions, and all the families of the earth will be blessed through you.
Seforno: It is when Yaakov's children will hit rock-bottom that they will spring back and flourish in all directions. 2
Tosfos ha'Shalem (3): Yisrael are compared to the stars and the dust. When they ascend, it is up to the stars. When they descend, it is to the earth. 3
Tosfos ha'Shalem (2, citing R. Efrayim, from Bereishis Rabah 69:5): Earth outlasts metal vessels. Also, Yisrael endure forever, and outlast all the nations.
See for example, how Yisrael proliferated in Egypt, from seventy to more than four million in the space of two hundred and ten years!
Hashem's salvation comes when things cannot get worse for Yisrael.
Oznayim la'Torah: This explains why Hashem associated 'the dust of the earth' with Yaakov's children - whose sins cause brought about the Galus; but the 'spreading' to Yaakov himself - since the Ge'ulah will come about via the Zechus Avos.
According to Maharal, what is the significance of "the dust of the earth" to which Bnei Yisrael are compared?
Maharal (Netzach Yisrael Ch. 57, p. 210): Earth particles are tiny, and all trample them; yet they can attain lofty qualities as a result. 1 A) Just as earth can receive water from Above, so too Bnei Yisrael receive the Torah from Above. B) Anything that is audible is revealed, devoid of internal content, while the inner hidden world is silent. In Daniel (2:32-40), the nations are compared to various metals, which produce a sound [when struck]. Yisrael is compared to earth, which does not make a sound, for they have an inner hidden dimension - and they will therefore outlast all the other kingdoms. C) All trample on the tiny earth particles, so too Yisrael; yet, because their physicality is less, they are more fit for the spiritual.
Also see Maharal (Gevuros Hashem, end Ch. 18, p. 84).
"West and east, north and south." In this verse, Yaakov was given "an inheritance without borders" (Shabbos 118a -b). What is the implication and significance of this blessing?
Gur Aryeh #1 (to 27:28): Yitzchak blessed Yaakov with the words "v'Yiten Lecha" (27:28) with the extra 'and' - meaning, 'May Hashem repeatedly give.' Yaakov was blessed without boundaries or limits; any blessing granted to him would lead to even more.
Gur Aryeh #2 (to Shemos 4:19): Yaakov was connected to the "'spring' of life," and a spring is unceasing. He was therefore blessed with ' an inheritance without borders;' and that, 'Yaakov Avinu did not die' (Ta'anis 5b).
Maharal #1 (Chidushei Agados Vol. 1, p. 54, to Shabbos 118a): Yaakov is known as Yeshurun (from the word Yashar - straight and honest), unencumbered by [pangs of conscience due to lack of] honesty.
Maharal #3 (Tif'eres Yisrael, Ch. 20): Yaakov was commanded to keep the Shabbos, for his trait was holiness, and his blessing was boundless - and the same is true of Shabbos. 5
Maharal #4 (Ohr Chadash, p. 154, on Esther 4:13-14): Yaakov and Esav are mutual foes; they fought even their mother's womb, for they could not be in the same space together. When one rises, the other falls. Therefore, Yaakov was blessed without boundaries - so that Esav could not contest them! Yaakov would be a flame, and Esav like straw (Ovadya 1:18).
Rashi: Not like Avraham, to whom Hashem promised only the whole of Eretz Yisrael (13:14,15) or like Yitzchak, to whom He promised "all these lands" (see 26:3).
See Oznayim la'Torah, who cites two opinions in the Midrash as to whether Hashem folded the whole of Eretz Yisrael underneath Yaakov, or whether it was just the Beis-Hamikdash. Either way, He only promised him that his descendants would spread to the whole of the land that was underneath him (not to the entire world, like Rashi).
This means, seemingly, that Chesed and Din have limits, and when taken in extreme they are harmful. Yaakov's trait was uprightness (Yosher), the balance of the two, which has no limit or opposition. (EK)
What did Hashem mean when he said, "and all the families of the world will be blessed through you"?
Targum Onkelus and Targum Yonasan: All the people in the land will be blessed due to you (and in your merit). 1
Rashbam: Refer to 12:3:2:4.
Seforno: Yisrael compared to the nations of the world are called Kohanim, 2 in which case they are the conduit which brings blessings upon the world.
Ohr ha'Chayim: Also Avraham received this Berachah. When he came to Aram, it was blessed. When Yaakov came to Egypt, it was blessed. "Uv'Zar'acha" - the nations will be blessed through Yisrael, who offer bulls for them during Sukos, and also in exile, the nations exist due to Yisrael.
Presumably, Rashi and Ramban explain similarly to above (Bereishis 12:3), i.e. people will say to their children 'be like Avraham!' (Rashi loc. cit.), and not only the people of Kena'an, but also the people all over the world will be blessed due to Avraham (Ramban to 12:2).
Hashem promised to Avraham "north, south, east and west" (above, Bereishis 13:14). Why did He change the order here to "west, east, north south"?
Riva, Hadar Zekenim: Avraham went to the south, so Hashem promised to him first the north, from where he came. Yaakov went to the east, so He promised to him first the west, from where he came.
Tosfos ha'Shalem (4, from Shemos Rabah 21:7): "U'Faratzta" is next to Yamah, to hint that in Yaakov's merit, Hashem will make Pirtzos (breaches) in the sea (i.e. split it).
Tosfos ha'Shalem (8): There are 13 Berachos here corresponding to the Shevatim, which will all inherit [in the days of Mashi'ach] from east to west (Yechezkel 48).
Malbim: Yehoshua will begin to conquer in the south from east to west, and afterwards to the north.
Why did the Torah use the word "v'Nivrechu"?
Refer to 12:3:2:4.
Why did Hashem tell Yaakov, "and all the families of the world will be blessed through you and your seed"? Regarding Yitzchak, He told Avraham (22:18) only that they will be blessed through his seed (not Yitzchak himself)!
Ha'amek Davar: The Berachah to Yitzchak was of tremendous wealth. Yitzchak himself had such wealth only with respect to the Pelishtim, but not with respect to all nations - therefore it says only "in your seed." Yaakov entered Lavan's house amidst utter poverty, and was blessed exceedingly through Hashem's Hashgachah. One who contemplates this will be astounded, and blessed through him!