What was special about Moshe's staff?
Seforno: Although it was made out of ordinary wood, Hashem sanctified it to act as a sign on behalf of Moshe.
Targum Yonasan (to 4:20): The staff, which Moshe plucked from Yisro's garden, 1 was made of sapphire taken from Hashem's precious Throne, and weighed forty Sa'ah.
Oznayim la'Torah: A Shali'ach who comes in the name of the one who sent him needs to bring some tangible proof of his Shelichus in order to be believed. 2 Hence, the people believed Moshe when he descended Har Sinai, because he was holding the Luchos. 3
Ramban (to 4:21, citing Midrash): The ten plagues were written on Moshe's staff, in the form of the acronym De'tzach, A'dash Be'achav
Refer to 2:21:1:1**. See also Ba'al ha'Turim.
Oznayim la'Torah: As the Gemara states in Yevamos 25).
Oznayim la'Torah: Even for what he brought along with them - the Torah Sheba'al Peh.
What was the significance of the staff? Why did Hashem instruct Moshe to take it with him on his Shelichus?
Seforno: It was a stick of rulership, the medium through which Moshe was able to override the laws of nature via his command.
Why is the specific term "Osos" - signs, used in this context?
Maharal (Gevuros Hashem Ch. 56, p. 248): "B'Osos (with signs) - This refers to the Mateh" (Hagadah Shel Pesach). This means the Mateh itself was the Os (as opposed to the miraculous plagues which it brought about). In fact, an Os is not the miraculous outcome, but rather the cause 1 of the miracle. 2 The Mateh itself was the Os, in that a mere wooden stick could bring about these great miracles.
Maharal (ibid.): We find that Osos are associated with the heavens, and Mofesim (wonders) with the earth (see Rashi to Devarim 13:2). The heavens are conceptualized as actively causing and giving, and the earth as passively receiving.
Maharal (ibid.): The miraculous outcome gets a different term, 'Mofes' (wonder). According to the Hagadah, the classic Mofes was the plague of blood; which was not just great abundance of an otherwise natural phenomenon (e.g. frogs), but a change in the earth itself.
Bearing in mind that Hashem had only performed one sign to date with the staff (see earlier, 4:4), to which signs (pl.) is Hashem referring here?
Rashi (to 4:21) and Ramban #1: To the signs that He would teach him shortly. 1
Ramban #2: When, earlier in the Parshah (3:20), Hashem referred to all the wonders that He would perform in the midst of Egypt, He presumably told him what they were. And those are the signs that He is referring to here. 2
