One of the most common objections to the teaching of the Doctrines of Grace is that it leads to antinomianism, that is, that Christians who focus too much on grace will quickly abandon God’s moral law and adopt an attitude of licentiousness. This is a very old debate (see Romans 6:1-14) but one that persists and probably always will. Follow the link, The Fear of Antinomianism, for an excellent discussion of where this fear gets it wrong.
Tullian Tchividjian on the same issue, Two Approaches to Realizing Radical Obedience.