Chazal tell us that the violation of Rabbinic commandments carries a more severe punishment than the commandments written in the Torah. Rav Yeruchom Levovitz zt"l explains the rationale behind this idea. The more one can relate to a concept or idea, the more it obligates him. It is easier for us to relate to the commandments and restrictions that were enacted by the Rabbanan, not only because the reasons behind them are more comprehendible to us, but also since those who instituted them lived in a more recent era than the one in which the Torah was given. Hence, these mitzvos obligate us to a greater degree and neglecting them carries a greater punishment.
Rav Wolbe (Da'as Shlomo Geulah pg. 201) writes that if mitzvos de'rabanon carry a greater punishment, it follows that their fulfillment brings greater results. The fact that we can relate to the mitzvah means that we can gain more from the mitzvah. Therefore, it is quite possible that we can gain more from Purim, which is de'rabanon, than we can gain from other Yomim Tovim even though they are mi'deoraisa. Additionally, the Arizal writes that Yom Kippurim means that it is "a day like Purim," i.e. Yom Kippur is secondary in greatness to the day of Purim. Another indication to the spiritual loftiness of Purim is the fact that a sefer in Tanach, a Mesechta of Gemara, and a plethora of mitzvos and customs are dedicated solely to this day.
The miracle of Purim was the very last miracle that happened to Bnei Yisroel which is recorded in Tanach. However, the uniqueness of this miracle is the fact that it occurred during a time of galus and hester panim. Therefore, we can glean from the day of Purim all that one needs to know to live and weather the days of galus and hester panim.
When Achashveirosh gave his ring to Haman, thereby allowing Haman to do to the Jews as he pleased, despite the hester panim the Jewish People were able to decipher that it was Hashem who brought upon them the decree of annihilation. This realization was more successful than the prophets in causing them to do teshuva. Moreover, when they merited the salvation through the hands of Mordechai, they literally felt Hashem's loving hand penetrating through the hester panim, which in turn prompted them to reaccept the Torah wholeheartedly.