The question has been raised many times why Hashem held a mountain over bnei Yisrael's heads and said accept the Torah or "po yihiyeh mitaschem" right after bnei Yisrael said "na'aseh v'nishma." You would think that Hashem would reward them for saying this not threaten them with death?! So I gave a possible pshat that maybe Hashem is telling them much more when He says that this will be their burial ground. Think about it. Hashem has just asked all the other nations if are willing to accept the Torah and they found something that they disliked and turned down the offer. Then He gets to bnei Yisrael who immediately proclaim "na'aseh v'nishma!" - before we even hear what you have to say we will do it! While this is an amazing display of bitachon and utmost devotion, Hashem is making sure that they know what they are getting into. The Torah represents the mountain and Hashem is showing them how difficult it is to conquer. But, Hashem says, if they don't accept it this will be their burial place meaning that they have nothing to live for and they will be as good as all the other nations- as good as dead.

I like that pshat

Similarly, if the bnei yisrael were forced to accept the torah, couldn't we claim today "Why should I keep the torah, my ancestors were forced to accept it, and now I don't want to?" Answer is because during Purim, we say kimu v'kiblu we established and accepted. We accepted the torah wholeheartedly.

During the period of miracles, the mountain was constantly "over our heads". C.f. seifer Shofetim. This is why Purim, the holiday that marks the hidden miracle, which is after the period of true nissim and at the close of the period of nevu'ah, also marks the completion of the acceptance of the Torah.

See my devar Torah on the subject.

There is also the concept that Israel was the LAST nation that G-d asked to accept the Torah.
All the other nations gave their excuses, and only WE were left.
The fact is that had we NOT accepted the entire creation would have ceased to exist as the purpose was for us to accept and live by the Torah. So to speak the plug would have been pulled on creation and everything reverts to "tohu vovohu".

In this why it was not so much of a threat but a simple fact. If we want to continue to exist we MUST accept and live by (and as Rahsi says LEARN) the Torah. If we stop for a second BANG everything is gone not as a punishment but simply because the purpose is taken out of creation......

Thank you.