Why, specifically here, does the Pasuk stress the distinction between the flocks of Yisrael and those of the Egyptians?
Ramban #1 and R. Bachye #1 (both in Pasuk 3): Due to the Egyptians' loathing of shepherds and sheep, they moved their animals far from the towns - most likely to an area bordering Goshen, where Yisrael's sheep were kept. In that case, the distinction between the two was even more marked. Moreover, pestilence is caused by a change in the air, and since they shared the same air, the distinction was even less comprehensible.
Ramban #2 and R. Bachye #2 (both in Pasuk 3): Because Makas Dever was the result of a change in the atmosphere, and there was no logical reason to differentiate between the sheep of the Egyptians and those belonging to Yisrael.
Seforno (in Pasuk 7): It demonstrated the Hand of Hashem, since nobody other than Hashem can guarantee life.
Rashbam: Refer to 8:18:1:1 and notes.
Riva (in 8:18): When the destructive angel is authorized to kill [en masse], he does not differentiate between good and evil. Therefore, a special differentiation was required.
Moshav Zekenim: Refer to 8:18:1:2.
Gur Aryeh (in Pasuk 14): The first set of Makos, De'tzach, brought Par'oh to realize that the Makos were from Hashem. But they still thought of the Makos as general, indiscriminate strikes at the world; they could not accept them as specifically directed against them by Hashem (Hashgachah Pratis). The Makos of Arov and Dever specifically excluded Goshen and the B'nei Yisrael (8:18 & 9:4), demonstrating Hashem's Hashgachah. 1
What are the implications of "v'Lo Yamus mi'Kol Livnei Yisrael Davar"?
Oznayim la'Torah (to 9:5): Chazal learn from here that if an Egyptian made a partnership with a Yisrael on his (the Egyptian's) animal, it survived the plague of Dever.
