What was the significance of the promise that he would join his fathers in peace?
Rashi: Hashem was telling him that he would live a peaceful life and that he would not suffer any of the travails which He had said earlier his children would suffer.
Ramban: He was telling him that it was the slavery and the subservience that he was destined not to suffer. The exile, he too experienced. 1
See R. Chavel's notes on the Ramban.
Seeing as Terach was an idolater, how could Hashem promise Avraham that he would join him after he died?
Rashi: This teaches us that Terach did Teshuvah before he died.
And what was the significance of "the good old age" that He referred to?
Rashi: This was an assurance a. that Yishmael would Teshuvah in his lifetime and b. that Esav would not sin publicly as long as he (Avraham) was alive. Indeed, Avraham died five years before his time, so that he should not see that happen, and Esav sinned on the day that his grandfather died.
רש"י: ואתה תבוא - ולא תראה כל אלה: מנין שהובטח לו לא לראות את כל אלה?
גור אריה: בפשוטו קשה מה ענין "ואתה תבא אל אבותיך בשלום" לכאן? אלא שהוא לא יראה את כל אלה.
רש"י: ואתה תבוא - ולא תראה כל אלה: קשה שגם באברהם נאמרה גירות?