These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Game Breaker: Actual teamwork. Coordination can result in the enemy team being completely disabled and held back. A few knights protecting decent archers can slaughter the enemy team with a hail of arrows while no one gets remotely close to them.
The overhead-look down exploit. Take a weapon like the Maul or Zweihander - extremely slow weapons that are capable of One-Hit Kill attacks, then run around staring at the ground while using the overhead attack. The animation tends to play after the attack, resulting in an effectively unblockable attack that will drain almost all of a player's stamina.
Recently fixed, though the zweihander is still a bit subtle.