As with all icons of the Eastern Orthodox Church, the Pentecost icon teaches theology and brings us into the reality of the event depicted. Below are a few reflections on the symbolism and meaning of this icon.
The Position of the figures represents harmony in which no apostle is better than another. The inverse perspective prevents the apostles who are near the back of the semicircle from being painted as smaller, which would happen in the rendering of most normal paintings. In actuality, they are depicted slightly larger, particularly Peter and Paul as the chief apostles. But even then, they are among equals.
There is also no discord or chaos, which contrasts with some of the Western paintings of this event, which can be a bit dramatic. There is no sign of appearing drunk, which they were accused of that morning. Everything is sober and harmonious.
While it is not a typical iconographic meaning, the towering architecture here reminds me of the Tower of Babel. Men of old thought they could reach divinity and become gods on their own by building a tower up into heaven. But God scattered them with different languages (tongues). Pentecost, however, shows that the Holy Spirit has been sent to unite all people in the new spiritual building of the Church, and through this new building we can reach heaven and become adopted children of God, no matter our race or tongue (language).
The Kontakion of Pentecost states,
When the most High came down and confused the tongues, / He divided the nations; / But when he distributed the tongues of fire / He called all to unity. / Therefore, with one voice, we glorify the All-holy Spirit!
Who are the figures in the icon? On the left, starting from the top, we have Peter, Matthew, Luke, and other disciples. On the right, we have Paul, John, Mark, and other disciples. Paul, Luke, and Mark were not part of the twelve disciples, but because they played a significant role in preaching the Gospel to all humanity they are depicted as being in unity and harmony with the twelve.
Of course, Paul was not present during this event, but the reality of Pentecost transcends time and space, and in some sense, Paul was later mystically joined to this moment. He is also included to show that he was in complete harmony with the other Apostles.
In many icons the four evangelists are depicted holding books and the remainder are holding scrolls. The books symbolize the writings of the four gospels, the scrolls show that even those who did not write lengthy gospels still proclaimed the gospel message to the world.
Why is the head of the semicircle of Apostles empty? Because it is the seat of our Lord Jesus Christ, who is invisibly present. As he said before ascending, “Behold, I am with you even to the end of the age.” He continues to keep that promise, being invisibly present in the church, guiding us through the Holy Spirit.
The twelve rays coming from the mandorla (semicircle at the top) and the tongues of fire, sometimes shown above each Apostle, represent the Holy Spirit coming upon all of those depicted. The twelve symbolize that there are a diversity of gifts given by the one Spirit of God. In the same way, each of the twelve Apostles are seated in slightly different ways: there is harmony, but not conformity. We are all given different gifts by the same Spirit, but none of us are meant to be a copy of anyone else. It is in the Church that we see diversity in its most beautiful harmony.
Something that stumps many people: who is the kingly figure at the bottom of the Pentecost icon? It is Cosmos. In the most ancient Pentecost icons, the crowd from Acts two was depicted at the bottom. However, that was quickly replaced by Cosmos, who personifies all people of all nations.
A 17th century description reads, “The man sits in a dark place, since the whole world had formerly been without faith; he is bowed down with years, for he he was made old by the sin of Adam; his red garment signifies the devil’s blood sacrifices; the royal crown signifies sin, which ruled the world [as a tyrant]; the white cloth in his hands with the twelve scrolls means the twelve Apostles, who brought light to the whole world with their teaching.” 
Ouspensky later writes that this icon is “an image of the inner life of the Church.” How so? It shows that Christ sits at the head of our Church, invisibly guiding us; we have descended directly from the Apostles; we have been given the gift of the Holy Spirit by God through the Apostles; we celebrate our own participation in Pentecost through Chrismation and the mysteries of the Church because these things transcend time and space; while we always work toward harmony and unity with one another, we have a diversity of gifts and talents and there is no forced uniformity; people of all nations, tribes, and tongues are united together in the new spiritual building of the Church that transforms us from being lowly earthly creatures to heavenly ones.
 N. Pokrovsky, The Gospels in Iconographic Records, St. Petersburg, 1892, p. 463. Quoted from Ouspensky, Lossky, The Meaning of Icons, 1982, p. 208
Further reading about this icon, including why the Theotokos is sometimes depicted at the center and why it is not proper to show the Holy Spirit as a dove in this icon: https://iconreader.wordpress.com/2011/06/14/pentecost-icon-as-an-icon-of-the-church/