The Gospel of John represents the maturation of the Gospel -- the tail end of the process of inspiration and reflection concerning God's message to humanity through Jesus Christ. In contrast, Acts is the start of this process, it is the record of the transition of Christianity from Jewish sect to international faith. This book records the birth of the church, and the numerous crises that threatened its existence in its early years:
-- the substitution of the Holy Spirit for the personal presence of Jesus Christ. This took some getting used to. Jesus and the Spirit were not interchangeable. The Spirit's presence was less tangible and his guidance less direct than Jesus in the flesh.
-- the relation to Jewish authorities, who at first tried to stamp out the remnants of the Galileans (Acts 8:1)
-- the perils of success: large numbers of converts, the need for organization and meeting places, the establishment of policies and doctrines
-- oh my God! the Gentiles are coming!
-- Paul and the evangelization of the Gentiles.
-- relations with the Greek churches in Asia Minor and Greece.
It is apparent from this book that God gave men a lot of discretion in building His church. There are accounts of the direct intervention of God: at Pentecost, when Philip was caught up in the Spirit to meet the Ethiopian eunuch, the conversion of Saul/Paul, Peter's vision of the unclean animals, etc. Yet there are also times when God stepped back and left it up to the elders to decide what to do next: such as the appointment of the deacons (Acts 6), and disagreement over the treatment of Gentiles (Acts 15). What we see over the course of the book is an increasing separation from the movement's roots in Judaism, and a slow redefinition in Gentile terms. This process is not completed by the end of the book, but Acts marks a way station towards the unique synthesis of Jewish and Greek worldviews that we find in John's Gospel.