What did Avraham suddenly discover about Sarah's beauty that he did not know before?
Rashi #1 and Targum Yonasan: Such was the level of Tzeni'us (modesty) of Avraham and Sarah that he never looked at his wife's beauty, and it was only as they were crossing a river on the way down to Egypt that he inadvertently saw how beautiful she was. 1
Rashi #2: He always knew how beautiful she was, 2 but was struck by the fact that the constant traveling did not detract from her looks.
Peninim mi'Shulchan ha'Gra: Esther was yellow, but grace was over her (Megilah 13a). This means that she was not naturally beautiful, but due to her Tzidkus, she appeared beautiful. Such grace would not persist if it could cause disaster, 5 but natural beauty would. Sarah remained beautiful even now, when it could lead to danger; this shows that she had natural beauty.
Either as she raised her dress to cross the water (Targum Yonasan), or when he saw her reflection in the water (Sifsei Chachamim).
Bava Basra (58a) says that everyone else was like a monkey compared to Sarah.
Also regarding Lot (19:2), the word "Na" connotes worry (lest Sodomites harm the guests). Gur Aryeh explains the parallel: In both verses, the word "Na" is not connected to the following words. It means, "The time has come to take heed."
Even though Esther's beauty caused Achashverosh to take her, that was Hashem's plan (PF).
QUESTIONS ON RASHI
Rashi writes: "Until now, [Avraham] had not recognized [her beauty]." Isn't it forbidden to marry a woman before seeing her (Kidushin 41a)?
Gur Aryeh: He had seen her face, but because the face is visible to all, one is not especially moved by it. [Now, due to an incident while travelling, Avraham realized his wife's beauty.]
Rashi writes: "to Egypt... brothers of Kush, where they are not accustomed to a beautiful woman." Mizrachi asks
Gur Aryeh: In Gerar, the inhabitants were wicked; upon their arrival, they immediately inquired as to Sarah's marital status (Rashi to 20:11).