One of the biggest contrasts in the play is that which mimics the Christian idea of justice and mercy, and how the two can or ought to be combined.  Shylock serves to play the part of the hard master who always demands justice without any kind of allowance for mercy.

The Duke of Venice himself points out the stony and indifferent quality of Shylock in comparison to the Christian idea of being merciful and kind.

There are other contrasts that are brought up in the play, surface qualities versus the deeper ones, particularly in the characters themselves.  A great passage that demonstrates this idea is as follows:

All that glisters is not gold;
Often have you heard that told:
Many a man his life hath sold
But my outside do behold:
Gilded tombs do worms infold.
(II.vii.65-69).

These are just a few, there are certainly more, the guide below will serve as a good reference.