1. The weapon
2. Your skills in various things
3. Your posture in combat
However, unlike dodge which takes into account how many people were attacking you, and penalizes you for being attacked by lots of people-- parry did not. That has now changed.
You will receive a small penalty for each person who is attacking you. The more people attacking you, the higher the penalty will stack, making it harder for you to parry.
This is a relatively minor tweak to the code but it could have implications for large fights, especially ones where the 'tank' would run in first and attack, drawing all the enemies to attack them, and then everyone else came in, went kamikaze and wiped the foes out.
That tank better have some ace armor, because they won't be parrying at the same insane level they were before. It was a good strategy and can still be implemented, but it will have to be done with more care and thought than it was previously.