A lot of the French knights ended up drowning / being trampled to death in their armor actually due to the mud essentially immobilizing any knight unfortunate enough to fall down in it. Also, the amount of French knights on the field should have been an advantage (they had many more knights), but the field was too small and the French were crowded too closely together to fight an effective mounted battle. These factors combined with many of the other factors mentioned previously in this forum made it a very bloody defeat for the French.
I have to disagree with some people on here though who seem to think that the English Longbowmen were some super elite fighting force. They were good at what they did, yes, (launching large volleys long distances), but when it came to melee they would stand little chance against mounted knights were it not for the many other factors playing against the French in this battle.