What is the significance of the phrase, "after Lot had separated from him"?
Seforno: Hashem did not want to inform Avraham this piece of news while Lot was there, for fear that it would go to the heads of Lot and his shepherds and would encourage them to rob their neighbors.
Rashi: As long as Avraham was in the presence of the Rasha, 1 Hashem declined to speak with him.
Chochmah u'Musar (Vol. 2, p. 370): Do not say that Lot was a Rasha. He disregarded his personal needs and abandoned his family to serve Avraham and learn Torah from him! Likewise, when it says about Shlomo that his wives veered his heart, it is not literally true. Rather, according to Lot's level, he did not decide properly, due to money. Hashem later showed him this
Why did Hashem begin here with Tzafon and by Ya'akov (28:14), with Yam (Ma'arav)?
Ba'al ha'Turim: Because when speaking to Avraham, He referred to the merit of the Korbanos 1 (Kodshei Kodshim) which are Shechted on the north side of the Azarah; whereas when addressing Ya'akov, He hinted to him that the sea (the Yam-Suf) would split in his honor. 2
Refer also to 28:14:151:1-3.
Why does the verse use the term "Va'Sh-m"? This always refers to Hashem and His Beis Din (see Rashi to Bereishis 19:24)!
Riva: Also here, Hashem's entourage wanted to kill Avraham due to the accompaniment of Lot, yet Hashem disagreed.
QUESTIONS ON RASHI
Rashi writes: "As long as the evildoer [Lot] was with Avraham, Hashem separated from speaking to him." This is difficult, because Hashem had already appeared to Avraham and said, "To your children I shall give this Land!" (12:7).
Moshav Zekenim: We can say [unlike Rashi] that Avraham was pained that Lot separated, so now Hashem consoled Avraham. Or, [Rashi] can say that earlier Lot was a Tzadik, and now he soured.
Riva: When Lot was with Avraham, Hashem spoke concisely, and now He elaborated.
Mizrachi: Avraham alone traversed the land until Shechem (to pray for Yaakov's children
Gur Aryeh disagrees; Lot was surely accompanying Avraham.