Mercy was a foreign concept to the Roman culture in which Jesus did ministry. In our culture, where so much energy and expectation goes into a sense of what others do and don't deserve- mercy is misunderstood, undervalued, and unappreciated.
Mercy- at its most basic level- is something undeserved. This alone made it radical back then, and makes it radical today. Here's the real kicker. Jesus- the Kingdom Reversalist- modeled and preached an even more radical expression of this already-radical concept of mercy.