What is "Ge'on Uzchem referring to?
Rashi: It refers to the Beis-Hamikdash. 1
Why does the Pasuk write here "Es Shemeichem ka'Barzel" and "v'Es Artz'chem ka'Nechushah", whereas in Devarim (28:19) it writes "v'Hayu Shamecha asher al Roshcha Nechoshes" and "v'ha'Aretz asher Tachtecha Barzel"?
Rashi: The punishment here is worse than the one in Ki Savo, inasmuch as 'The sky will be like copper' (as stated there) implies that it will sweat (drizzle rain) like copper sweats, and 'the earth will be like iron', that it will not sweat (to produce a humidity that spoils whatever grows); whereas here, 'The sky will be (dry) like copper and the earth moist like iron', which implies that not a drop of rain will fall and any thing that happens to grow will be spoiled by the humidity of the earth. 1
QUESTIONS ON RASHI
Rashi writes that the punishment here is worse than in Ki Savo. It says in Bava Basra (88b) that Hashem is unlike (more benevolent) than people - Hashem blessed them with all 22 letters (from Aleph (Im) to Tav (Komamiyus), and cursed them with eight (from Vov of v'Im until Mem of Avonam), and Moshe cursed them with 22 and blessed them with eight!
Moshav Zekenim: Even though [Hashem was careful that His Brachos span more letters than His curses, and Moshe was not], His curses were harsher.
Toldos Yitzchak (42): Hashem blessed them from Aleph to Tav, to hint to them that the Brachos will be only if Yisrael fulfill the Torah from Aleph to Tav. 1 He cursed them from Vov until Hei (of b'Yad Moshe. These letters comprise half of His name - "Imo Anochi b'Tzarah" (it is as if Hashem's name is not complete when Yisrael suffer).
If so, also regarding this, Moshe was more benevolent. Perhaps his curses are only if Yisrael transgress the Torah from Aleph to Tav! (PF)