Actually, the key word is the giving of the Torah. Before the 6th of Sivan, we were privy to the wisdom of the Torah, but it did not belong to us. We can compare the Torah to a magnificent palace. Before the Sinai event, we were given unlimited access to tour it's rooms and enjoy it's beautiful architecture and artwork; at Sinai, we were handed the keys and ownership of the property. 

The Gemara related a fascinating episode which powerfully illustrates this point. Rabbi Eliezer the Great disagreed with Rebbi Yehoshua and the other Chachamim of that generation regarding the susceptibility to impurity of a certain type of stove. Rabbi Eliezer vigorously defended his position, but to no avail-- the Rebbiam refused to accept his position. At a certain point, a heavenly voice reverberated in the study hall: "Why do you argue with Rebbe Eliezer? The halacha is always his position!" Rebbe Yehoshua then arose and exclaimed, "The Torah is not in heaven!" The Torah was given to us on Har Sinai, and therein it is clearly stated that the majority prevails. Since the majority of the Chachamim concurred with Rabbi Yehoshua's opinion, the opinion of supernal beings-- indeed the opinion of God Himself-- is immaterial. Amazingly, the halacha was established according to the majority opinion-- and Rebbi Eliezer the Great was excommunicated for refusing to accept this ruling.

This, however, does not completely explain the significance of Shavuos. Does this Chag boil down to bragging rights? Does it really matter who "owns" the title to Torah? As long as we're permitted to study Torah, would it be such a great cause of panic if God actually had a say in determining halacha? King Solomon likened the revelation at Sinai to our wedding day-- the day God chose us from amongst the other nations and wedded us. On the occasion of His marriage, the groom, who was (and remains) madly in love with His bride, could find no greater downpayment to give then the Torah, His most precious possession, His crown jewel, His joy and pride. Furthermore, He didn't suffice with making this treasure available for our use; He lovingly gave this gift completely and unconditionally. This is why Shavuos is so special. It commemorates the day when God gave us the most precious jewel in His treasure house. "Fortunate are the people whose lot is thus!" God's love for us drove Him to gift us His most precious possession. Now, how do we reciprocate? We have the two days of Shavuos to contemplate the response to this question...