To begin with, Shavuot is wonderful specifically because it is unencumbered by considerations beyond the laws of Yom Tov. Indeed, it is Yom Tov, pure and simple. It's a day given totally over to rejoicing before God. The Torah commands us, ושמחת בחגיך. The Rav זצ"ל always pointed out that the ultimate joy a Jew can experience is to stand before God, ושמחת לפני ד'. On other holidays, there are all kinds of accessories to help us do that. On Shavuot, in the afterglow of Pesach and 49 days of spiritual preparation through the Omer period, we are bidden to just experience God's presence and rejoice; physically and spiritually. The paucity of ceremonial is, indeed, its power.
In addition, Shavuot has a mitzvah. We are bidden to re-experience Revelation and to sit and study Torah. What is better, more sublime. more exalted and more spiritually intoxicating than meeting God, the Creator of the World? As R. Hayyim of Volozhin writes in Sefer Nefesh ha-Hayyim (4, 3), attaining insight into the Torah is a personal act of Revelation. And we do it through a meeting of physicality and spirituality (רוחניות וגשמיות).
That is why, for me, Shavuot is not only unforgettable. It is my absolutely favorite Hag.