Notice two things about the miracle of Pentecost: its nature and its participants.
The Miracle's Nature: The miracle temporarily "healed" the ailment God imposed at Babel. There, God divided the languages of mankind, inflicting on it an impediment to communication (see Genesis 11:1-9). Suddenly, relationships became much more difficult to establish and maintain. Mankind scattered.
Communication is a two-way street, involving a source and a receiver. These are what we call the speaker and the hearer, respectively. By changing speaker and hearer, the miracle brought source and receiver together, where normally they would remain distant. The disciples spoke languages in which they were untutored. Members of their audience heard the disciples "speak in his own language" (Acts 2:6). Communication took place.
As miracles go, this is a "strange" one. It did not involve healing the blind, deaf, or lame en masse; it did not involve the wholesale casting out of demons. Compared to the plagues God sent on Egypt or to Jesus' raising of Lazarus from the dead, the Pentecost miracle was not dramatic. Nevertheless, we will see that it was significant.
The Miracle's Participants: The miracle involved Jewish speakers of a substandard dialect and Gentile hearers from around the world. The disciples were Galileans. By virtue of the distance separating them from Jerusalem, Galileans spoke a different dialect of Aramaic than that spoken in Jerusalem. Like many dialects, theirs was what linguists call a "shibboleth," a term they get from Judges 12:6. A shibboleth is a speech pattern that identifies the speaker's background. In the disciples' case, it marked them to be what the Jerusalem leadership considered uneducated and low class. As an analogy, one could compare the Galilean dialect to "cockney" English—also a shibboleth. The dons of Oxbridge look down on those who are "unfortunate" enough to speak cockney. That is how the effete Jewish elite in Jerusalem reacted to the Galilean dialect. Everyone who heard the disciples knew they were from Galilee. Their audience was dumbfounded that these untutored fellows could fluently speak other languages.
Notice that they mention "Jews and proselytes." The cosmopolitan audience was not composed merely of Jews who had traveled from abroad for the holy day, but also of Gentiles converted to Judaism—that is, proselytes. Unlike typical Jews today, pre-Diaspora Jews (before AD 70) were dedicated missionaries. Christ Himself refers to their evangelistic zeal: "You travel land and sea to win one proselyte" (Matthew 23:15). Over the years, the Jews—like evangelicals today—had carried their religion everywhere. Paul preached the gospel in synagogue after synagogue throughout the Roman Empire. There were synagogues in the Parthian Empire as well; Peter, when he served God in Babylon, certainly frequented them. Judaism had reached the Far East by Christ's time and perhaps the distant West as well.
Pentecost's was a miracle of language. It showed Peter what Pentecost was all about: God had enabled communication between Himself and mankind. He had made it possible to build a relationship between God and man and between man and man. Even if human civilization had reached the end of its rope—suffering the judgment of God, as Joel apocalyptically describes it—"whoever calls on the name of the LORD shall be saved" (Acts 2:21).
Most importantly, Peter understood that this new level of communication included the Gentiles. This is why Joel's words struck home to him on the Day of Pentecost. He knew that the word "whoever" included the Gentiles scattered about in the audience. He preached good news to them: They now had access to God's salvation.