One thing I find amazing about this passage is that it almost didn't make it into the Bible. In fact, before the canon of Scripture was finalized in the late 4th century, some versions of John's Gospel didn't include this story.
St. Augustine tells us (cf. De coniugiis adulterinis, 2, 6) that many early Christians were afraid of keeping this story in John's Gospel since it showed Jesus as being so merciful that they thought it might lead people to think that the reality of sin wasn't as big of a deal as the Church made it out to be.
Apparently, the Holy Spirit wasn't concerned about this, since He made sure the story was kept in there.
Mark Shea's take on this story in his recent article on The Sixth Commandment is well worth a read. Check thou it out.