Previously, I'd thought of this exchange between Barabbas and Jesus only as a representation of the exchange between guilt and innocence (aside, of course, from the recounting of an actual event). In the same way that innocent Jesus takes the place of guilty Barabbas, He also takes the punishment that should be ours.
Considering the passage more closely, I realized that there's a bit more depth than that; there are also specific contrasts between the characters of these two figures. In John 18:40, the Evangelist comments "Now Barabbas was a robber," and this contrasts with Jesus, Who gives freely (the feeding of the five thousand, for example). In Mark 15:7, Barabbas is described as one of "the rebels in prison, who had committed murder in the insurrection." Jesus is the exact opposite of these attributes, too. Instead of rebelling, He kept the law perfectly (Hebrews 9:14), and rather than murdering, He creates life (in Acts 3:15, He is called the Author of life).
My other realization, looking at the accounts from a different perspective, is that the crowd is representative of our sinful human nature. In the same way that the crowd calls for Barabbas and rejects Jesus, we would cling to the darkness of sin and shun the Light of righteousness.
