I'm so used to reading these passages and accepting them as matter of fact when, of course, there is nothing matter of fact about them. The "simple" fact of Jesus' baptism carries a wealth of information that affects us deeply, as we can see. This not only helps me understand the great power of our own baptism, but how intricately interwoven everything is in the balance of salvation history and our own relationship with Christ.
Jesus' coming up out of the water (anabaino) is answered by a coming down (katabaino) of the Spirit from above. According to the Old Testament, sin creates an insuperable barrier, distancing humanity from the holiness of God (see Isa 59:2). God would "come down" to his people only after they had been cleansed of impurity (Exod 19:10-11). The Spirit's descent upon Jesus foreshadows his descent upon the Church at Pentecost, after sin has been removed by the cross.
The whole cosmos is impacted by Jesus' act of humility. The heavens are not gently opened but torn asunder--a sign that the barrier between God and man is being removed. Israel had pleaded for God to intervene decisively in human events: "Oh, that you would rend the heavens and come down" (Isa 63:19). Now that plea is answered! The same verb "tear" will reappear at a crucial point near the end of the Gospel, when the curtain of the temple is torn from top to bottom at Jesus' death (Mark 15:38), completing the reconciliation of heaven and earth that began at his baptism.