Chivalry/heroism - In the Middle Ages the knight were deemed to represent the height of chivalry and heroism. Monty Python mocks this chivalry and heroism by presenting Arthur's knights as the antitheses of their medieval counterparts.
Violence - What makes violence funny? How does Python make us laugh at a limbless knight or at slaughtered wedding guests?
Reality - Are Arthur and his knights really searching for a grail or are they a bunch of delusional role-players? Note the coconuts, the bad uniforms, the dead historian, the police cars, etc.
Religion/Christianity - This point should also be obvious; note the monks banging their heads with boards, the appearance of God, and the reading of the Bible in re: the