One of the more amazing things about this story is how little it is used by the early church fathers. There's a good reason for that: they devoted themselves to explaining the more difficult portions of the Scripture. This one is blazingly obvious. But perhaps we can learn something—not from what was said, but from what was not said.
Jesus declines the reply
When the lawyer asks his question, Jesus turns it around. Perhaps he felt this one a bit too obvious—or perhaps he wanted this lawyer to convict himself.
Jesus did not define "neighbor"
The lawyer was probably looking for a definition with distance in it. Jesus not only didn't do that, he didn't give him anything. He didn't even say, "Everyone." Why? So that the spirit of legalism shown by the lawyer would not define Jesus' answer; Jesus will use his method, not the lawyer's.
Jesus didn't condemn the man who was robbed
It was a rather dumb thing to do. The road to Jericho is a perfect setting for robbery by ambush, and well known for it. He could have said, "It was this guy's own fault." But he didn't.
Jesus didn't give any excuses for those who walked by
Walking by seems perfectly reasonable. Jericho is a city of priests. This could have been a trap; the priest and Levite would have been ceremonially defiled; there was the hatred of Samaritans; there was the expense; and after all, neither of these guys was a doctor.
The lawyer didn't use the word "Samaritan"
He said instead, "the one who had mercy on him." God is no respecter of persons.
The man with the cardboard sign
The test of your faith is not the hypothetical "what would you do if you saw." but rather "what do you do when you see." The man with the cardboard sign by the road: it might be his fault he's there. You might have a hundred good excuses—fear, it's late, he'll probably buy drugs with the money, he looks awfully dirty. But I ask you this: when you face your Lord on Judgment Day, which of those excuses will you offer to Him? And how will you explain to Him that you didn't get the point of the most obvious parable He ever told?