Where exactly was the ladder located?
Rashi: The foot of the ladder was in Be'er-Sheva (in the south of Yehudah), the top in Beis-El (in the north of Binyamin) and the middle in Yerushalayim (in the north of Yehudah, where it borders Binyamin).
Ramban #1: The foot of the ladder was in Be'er-Sheva, and the top was in Yerushalayim (which Yakov called Beis-El, alias Luz).
Ramban #2: The foot was in Yerushalayim, and the top was in Beis-El.
Where was Yakov?
Ramban: He was at the foot of the ladder, i.e. in Be'er-Sheva or Yerushalayim. 3
Beis ha'Mikdash refers to the place where it would be built, or the Beis ha'Mikdash in Shamayim.
And this was the Kefitzas ha'Derech mentioned in Chulin (Daf 91b). See Ramban, who strongly challenges this explanation.
These are the opinions of Rebbi Yosi ben Zimra and R. Yehudah in the Midrash, respectively, unlike R. Simon. (The latter conforms to the Gemaros in Chulin, Daf 91b) and in Sanhedrin 95b).
What is the significance of "Mah Nora ha'Makom ha'Zeh"?
Ramban: We learn from here that when someone prays in Yerushalayim it is as if he is praying before the Kisei ha'Kavod, as the Gates of Heaven are open to accept his Tefilos.
Moshav Zekenim: Luz became desolate, for they were living in a Kodesh place. Yakov feared that also he would be stricken, for he slept there.
Tosfos ha'Shalem (6): In this place Neshamos rise, the Sanhedrin sits, the Beis ha'Mikdash will be, the Aron will be, and it is the gate of Tefilos. The words allude to these, e.g. Nora is Aron backwards.
Tosfos ha'Shalem (7): "Mah Nora ha'Makom ha'Zeh" refers to Bayis Rishon. "Ein Zeh Ki Im" refers to Bayis Sheni. "V'Zeh Sha'ar ha'Shamayim" refers to Bayis Shelishi.
What did Yakov mean when he described his current location as "the Gateway to Heaven"?
Rashi #2: The Heavenly Beis-ha'Mikdash is located exactly opposite the Beis-ha'Mikdash on earth. 3
Bechor Shor: It is the place where the angels enter and leave in front of Hashem and to His house, to do His mission.
Rashi: That is the spot from which Tefilos (and Korbanos - Ramban) ascend to Heaven.
Ramban: According to one explanation in the Midrash (Refer to 28:17:2:3), the 'Gateway to Heaven' mentioned here refers to Be'er-Sheva.
Tosfos ha'Shalem (16): Since Rashi says that the Beis ha'Mikdash jumped to here, he must say so also about the Beis ha'Mikdash above. Ohr ha'Chayim
Why was Yakov afraid?
Seforno: He was afraid on account of his having erroneously slept there. 1
Ohr ha'Chayim, Ha'Emek Davar: He felt fear by himself (without a reason), due to the awesome [Kedushah] of the place. 2
How did Yakov know that this is Beis Elokim?
Da'as Zekenim: He saw that the angels ascend, descend and remain there.
Ha'Emek Davar: He understood from the fear he felt that this place is proper for Tefilah like Beis Elokim.
Why did Yakov say "Ein Zeh [Ki Im Beis Elokim]"?
Tosfos ha'Shalem (15): It is not a dream. Rather, it is clear truth - this is Beis Elokim.
Tosfos ha'Shalem (9, from Bereishis Rabah 69:7): "Ein Zeh" refers to the time of Churban. 1
Tosfos ha'Shalem (7): "Ein Zeh" refers to Bayis Sheni [in which five of the miracles of Bayis Rishon were missing].
Also Kinah 14 that Ashkenazim say on Tish'ah b'Av alludes to this. (PF)
QUESTIONS ON RASHI
Rashi writes that Hashem performed with Yakov Kefitzas ha'Derech (Refer to 28:17:2:1:1) to speed up his arrival in the Beis-ha'Mikdash. Why did He not stop him when he passed it the first time?