Targum Yonasan: His name was Zagnugi'el - Moshe's Rebbe.

Ramban (citing Midrash): It was Micha'el - the Mal'ach ha'Go'el. ¹

Maharal (Gevuros Hashem Ch. 23, p. 98): One opinion in the Midrash is that it was Micha'el - for it was followed by Giluy Shechinah (3:4), and Micha'el is 'first to the Shechinah.' Or, it was Gavri'el - who is appointed over Din, and this vision was with Midas ha'Din (see 3:1:1.2:2). ²

Rashi: Because a bush is a symbol of lowliness, and Hashem was therefore demonstrating that He is with Yisrael in their suffering. ¹

Da'as Zekenim, Hadar Zekenim: Because it is impossible to make idols from a bush.

Maharal #1 (Gevuros Hashem Ch. 23, p. 100): Any place is fit for the Shechinah to be revealed, even a thorn-bush. They grow everywhere; this teaches that Hashem's presence is not limited to specific or important places.

Maharal #2 (ibid.): Because this entire vision was for Yisrael's sake, Hashem appeared to Moshe in a place that reflected Yisrael's current condition. ²

Maharal #3 (ibid.): The thorn-bush is tough; like the difficult Midah currently in force upon Yisrael.

Maharal #4 (ibid.): The thorn-bush is used as a hedge around a garden. On the one hand, it stands apart from the garden; yet it also completes it. This reflects Yisrael's function towards the nations.

Maharal #5 (ibid. p. 101): The thorn-bush is tough; and it has both thorns and blossoms. So too is Yisrael a stubborn nation (Kesheh-Oref). There are big Resha'im, as well as big Tzadikim who stubbornly cling to their faith.

Maharal #6 (ibid.): The thorns on the Seneh face inward. While a person can insert his hand without detecting them; when he tries to bring it out again he will feel them! The Bnei Yisrael were originally attached to the nations; just like a fruit is at first surrounded by a peel until it falls off. Our arrival in Egypt did not need open miracles, for we were still attached to them. Our departure - as we detached from the nations and became attached to Hashem - required open miracles recognized by all. ³

Maharal #7 (ibid.): The Seneh has leaves in sets of five; we were redeemed in the merit of the three Avos, Moshe and Aharon. Exile is represented by the number 4 (e.g. four directions); and redemption, by the number 5 (the central point which unites them). ⁴

Maharal #8 (ibid. p. 102): The word "ha'Seneh" has a Gematriya of 120, the same as the lifetime of Moshe. The Seneh grows in an arid, water-deprived climate; so too was Moshe drawn from the water (i.e. from all physicality ⁵ ).

Rashi (to 3:12): To show Moshe that just as the bush carried out Hashem's will and did not burn, so too, would Moshe carry out Hashem's will and no harm would befall him.

Seforno: The bush symbolized the Egyptians. ¹ This was a sign that, because they refused to submit to Hashem, they would suffer the ten Makos, one Makah after the other - and not succumb. ²

Seforno: The Torah there is speaking after Matan Torah, where it explains how the people went back to their wives, whereas Moshe remained with Hashem. ¹ It is from that time on that he attained a higher level of prophecy than all other prophets. Until then, he, like all other prophets, was unable "to look at Hashem." ²

Maharal (Gevuros Hashem Ch. 23, p. 98): Hashem was in fact about to reveal His Shechinah to Moshe directly (3:4). This vision (of the burning bush, appearing via an angel) was but a preparatory stage, to cause Moshe to turn towards the mountain. As such, it was not fitting that it be done by Hashem directly.

Hadar Zekenim #1: It was so Moshe would not fear the fire on the day of Matan Torah.

Hadar Zekenim #2: It was a sign that Yisrael are like fire, and the Nochrim like thorns. Yisrael will not consume them (in this world), nor will the latter extinguish Yisrael's fire, i.e. Torah.