Joel promises a time when all barriers to the office of prophet will be removed. God's spirit will be accessible to all. No more will people be dependent on priestly or prophetic mediators to tell them what God is doing and what decisions they should make in their lives. This word is similar to the hope expressed by Jeremiah (31:31-34) that God will write the law on their hearts so they will no longer be dependent on another to interpret for them. They will all know the Lord, from the least of them to the greatest.
Obviously, this time of widespread prophetic illumination has not yet finally arrived, despite the breakthrough in Acts 2. Though some may claim to have divine inspiration, others who claim the same spirit will forcefully disagree about what God is saying and doing. This is a promise for the end-time, a promise that is yet to be fulfilled. In the meantime, some are wiser and more spiritually connected than others, and we ordinary believers are dependent on them for insights. With such a variety of claims to be God's spokespersons, it remains a problem for us, as it was in biblical times, to sort out the true from the false prophet.
It is also true that we should be careful not to set limits concerning who can speak for God and who cannot. It may be that the spirit has indeed inspired certain people with prophetic wisdom, even though their churches have not seen fit to recognize that the prophecy of Joel may already be at work in places we do not control.
The writer of Acts referred to this passage to help explain the phenomenon that occurred on the Day of Pentecost (Acts 2:17-18). The spirit descended on the people gathered there so that they spoke many different languages and all understood in their own language. For Peter, in Acts, this was a sign that in Christ the "last days" had come.
I will pour out my spirit on all flesh;
your sons and your daughters shall prophesy,
your old men shall dream dreams,
and your young men shall see visions.
29 Even on the male and female slaves,
in those days, I will pour out my spirit.
New Revised Standard Version Bible: Anglicized Edition, copyright © 1989, 1995 National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved worldwide. http://nrsvbibles.org