We've been going through the Sermon on the Mount at our church homegroup, and it's just occurred to me (a bit late in the day, I know) that what the Sermon is not doing is instituting newer, harsher rules for the Christian to live by, but showing what a life lived in excess of the Law looks like. That is, what Jesus seems interested in doing is showing the difference between the life lived by the Law and the life lived through grace. It is because the Law is actually easier to fulfill in the flesh (Philippians 3: 4-7) that what Jesus is saying looks so much harder by comparison. The Sermon is about the freedom of the Christian, of whom Luther says he is the most bound and the most free among the people of the world, because to be free from law but bound by love is totally opposed to our flesh (by which I don't mean our physical bodies, but our flesh-nature, selfishness, greed, lust, etc).
Anyway, that's my tuppeny worth.