What does "Kohen Midyan" mean?
Rashi, Targum Onkelos and Targum Yonasan: It means the ruler of Midyan. 1
Ramban: It means the priest of Midyan. 2
See also Rashi to Shemos 19:6. Perush Yonasan (citing the Aruch) - Targum Yonasan's translation ('Oneis de'Midyan') means the donkey of Midyan, a derogatory title ascribed to him because he was a priest of Avodah Zarah.
Ramban: The Pasuk does not mention his name here, because 'the priest of Midyan' was the title by which he was known. See also Ba'al ha'Turim.
Who was this Kohen of Midyan?
Why does the Torah need to tell us how many daughters Yisro had?
Maharal (Gevuros Hashem Ch. 19, p. 89): This tells us the virtue of Tziporah; the seventh one is always sanctified.
Yisro took in Moshe; just like Lavan took in Yaakov. Why was he not blessed with large flocks of sheep, like Lavan?
Oznayim la'Torah: Unlike Lavan, who loved money and who only took in Yaakov because of his prowess at shepherding - even going so far as to sell him his daughters against seven years of shepherding, Yisro displayed no specific interest in wealth. He gave his daughter to Moshe without any financial stipulation, and was even placed in Cherem, losing what was probably a lucrative position as leader or priest of Midyan - in his search for the truth. Consequently, Lavan received the sheep that he craved, whereas Yisro found - the truth for which he searched; he found Hashem, whom he adopted as his G-d. 1
Oznayim la'Torah: Hashem gives remuneration for each person in the 'coinage' of his choice.
QUESTIONS ON RASHI
Rashi writes: "'Kohen Midyan' - [I.e.] the greatest among them." Why doesn't Rashi give the usual explanation, that 'Kohen' means an idolatrous priest?
Gur Aryeh: Rashi tells us that at this point, Yisro must have already abandoned the service of idols; 1 the verse would no longer call him a 'Kohen' in that sense. But despite being put into Cherem by his people, he maintained his 'great' status, for 'we promote, and do not demote' (Ma'alin v'Ein Moridin).
Rashi writes: ".. But he had abandoned idolatry." The Midrash writes that Yisro had served every [form of] idolatry in the world! Why did he do that? What does this teach us?
Maharal (Gevuros Hashem Ch. 19, p. 90): Each form of idolatry worships but one individual power; but Yisro was searching for something more. He was looking for a power that includes everything - and therefore at the end he converted, to serve Hashem. 1
Idolatry is in essence worship of one of the forces that exist in creation (see Rambam, Hilchos Avodah Zarah 1). Yisrael believes in Yichud - that we must serve only the Master of all forces. Also see Maharal (Chidushei Agados Vol. 1, p. 36, to Shabbos 56b) as to why Yarav'am made two idolatrous calves.
Rashi writes: "... But he had abandoned idolatry, and they excommunicated him." How can this be shown?
Gur Aryeh #1: The text indicates that the shepherds would harass Yisro's daughters on a regular basis. Would they do so to the high priest of Midyan? He must have been in Cherem, for abandoning their faith.
Gur Aryeh #2: Why would the high priest of Midyan send his own daughters for the lowly work of tending the sheep; could he not hire shepherds? It must be that he was in Cherem, and no one would agree to work for him.
Maharal (Gevuros Hashem Ch. 19, p. 87): The following verse says "va'Yegareshum" (2:17) in the masculine gender; whereas we would expect the word 'va'Yegareshun' in the feminine. The shepherds did not harass them simply as men taking advantage of defenseless women, but rather because of the Cherem upon the family. 1
Implying that even had Yisro's shepherds been male, they would have been harassed.