My baseline for "don't be shitty" would be the non-aggression principle. Do not initiate coercive force against other people. It's the basic Kindergarten rule of, "don't hit people and don't take their stuff" using big adult words. Beyond that, try not to be dishonest or verbally abusive. Cut other folks the same slack you'd want them to cut you. The Golden Rule covers the whole concept pretty well, too.