In the merit of righteous women of that generation, Israel were redeemed from Egypt. (Midrash Rabba 1:12)
What is the textual basis for this midrash?
The redemption from Egypt occurred under Moshe's leadership. But Moshe would never have accomplished what he did, were it not for a number of righteous women who protected him at each stage of his early life. Yocheved hid Moshe from the Egyptians; Miriam guarded him on the river and made the crucial suggestion which allowed Moshe a Jewish upbringing; Pharoah's daughter protected and raised Moshe in spite of her society's expectations; and Tziporah was quick to circumcise their son and save Moshe from whatever sudden illness he had contracted. Were it not for the righteousness of these four women, the redemption could not have happened the in way it eventually did.
So far this is a practical argument. Due to the righteous deeds of these four women, Moshe was able to lead the departure from Egypt. But if God had wanted, couldn't the redemption have happened without Moshe? If these four particular women had been evil, and all the other women righteous, would this have prevented the redemption? I don't think so. So what is the importance of their personal righteousness?
From a literary perspective, the fact that all four women in the story show exceptional courage and resourcefulness cannot be an accident. We could have expected one or two heroines - but for every single female character to be a heroine is extraordinary. This is especially striking in light of the way Israelite men (four of them) are depicted in the story. Amram comes off as pretty remote from the story; he is not recorded as having helped save his son Moshe. Another Israelite was beaten or killed by the Egyptians, but that's all we know of him. The last two Israelites depicted get in an fistfight with each other. Amid this pretty mediocre behavior by the men, the constant heroism of the women is all the more surprising. It's hard to believe these four examples of righteousness are exceptions and not the rule (especially because three of them are not named). These four women say something not just about themselves, but about the behavior of women of the time in general.
(I hear the core of this explanation from R' Yaakov Medan.)
Update: I forgot the midwives, who would have made the argument even stronger.