They make the CofW a covenant of Graciousness instead of a covenant that has a debt attached to it. It messes up the fact that the Messiah had to fulfill the Covenant of Works. It matters in the fact that we are imputed with something that we didn't earn in the beginning but Christ earned it for us.
Nikki this may help a little.
A Working Federal Vision Summary from a 1689er - The PuritanBoard
P.S. I am not saying Piper is headed into Federal Vision problems.