It's not a lifetime commitment. It's only three minutes of your life that you are devoting to your social partner. Therefore, as a general rule, criticizing your partner is usually a gauche faux pas on the dance floor. But this thread is devoted to those exceptions where it's at least okay. I'll start: When (s)he hurts you. Bad technique is rampant. But when it causes you pain, the safety margin line has been crossed. This would be the time to nip it in the bud! If it's painful, then it is wrong. You have a right to speak up for yourself to end the hurtful actions even at the expense of your partner's ego. Be polite at first. But when that fails, being blunt is better than not stopping it.