Karma is designed to be an accurate representation of your recent in-game behavior and stability as a player based on your activities in the game.
We do not want players to be forever plagued by an unavoidable AFK or heat-of-the-moment outbursts of bad behavior. We also don't want great karma players to be able to build up karma and then troll without consequences.
By design, Great Karma is very difficult to get and requires you to consistently receive thumbs up from a majority of the players you play with, including the enemy players. It also requires you to avoid most, if not all, thumbs down. Going AFK or Dodging a match will quickly lower your karma and remove the Great Karma level.
Having Great Karma will increase your Glory gain by giving you bonus Glory at the end of your matches!