Commentaries:
It is interesting that Peter does not say that this was the fulfillment of what Joel prophesied. This is because much of Joel's prophecy was not yet fulfilled. It is easily seen that Peter understood this. He knew that it was only the beginning of the fulfillment of Joel's prophecy because God's Spirit was not being poured out on all flesh. The sun and moon had not changed their appearance, and not everybody that "called on the name of the Lord" was being saved. This is something similar to what happened when Jesus quoted Isaiah 61 in Luke 4, as He began His ministry in Nazareth. He left out parts of the prophecy. Likewise, Peter recognized that the events of Acts 2 only began to fulfill Joel's prophecy.
John W. Ritenbaugh
Pentecost and the Holy Spirit
Here, on the day of Pentecost, many people saw and heard what was happening, but who stands up and speaks? Apparently, all the apostles spoke at one time or another, but it is only Peter who is singled out in Scripture as the one who gave the keynote address.
John W. Ritenbaugh
Unity (Part 2): God's Pattern of Leadership