Why did He say "I cut off all your enemies from in front of you"?
Rashi: This is why you thought to build the Beis ha'Mikdash, based on Devarim 12:10-11.
Malbim: You do not have the absolute serenity required to build the Beis ha'Mikdash. Your enemies are cut off in front of you - but not in front of Yisrael, and they were not eradicated.
What is "k'Shem ha'Gedolim"?
Rashi (citing Pesachim 117b): Yisrael will say [at the end of the third Berachah after reading the Haftorah] 'Magen David' (similar to 'Magen Avraham').
Radak: It is like the great kings - "v'Yatza Shem David b'Chol ha'Aratzos" (Divrei ha'Yamim I, 14:17).
Malbim: Your kingship will be permanent, just like the name of the Gedolim endures.