Negative moodlets on the Sim related to anger are 50% more effective.
Sims with the Hot-Headed trait will become "Very Angry" with the other Sim right away if the other Sim performs a Mean interaction with them, as opposed to thinking the other Sim is "Impolite", "Rude" or "Unforgivably Rude". Some Mean interactions like "Greet Rudely", "Accuse of Being a Diva" (if the Hot-Headed Sim does not first have the Diva trait) and "Slap With Glove" bypass this and cause them to feel insulted instead of being angry.
Sims with Hot-Headed trait do not receive 'Humiliated' moodlet when a mean action is committed on them (like being yelled at, been argued with or being slapped by another sim).
Hot-Headed Sims will always retort when argued with, never being distressed unlike other Sims. This trait is shared with Diva Sims, who will also proceed to automatically slap the Sim arguing with them.
Hot-Headed Sims are still subject to an awkward reaction when an Argue or Insult interaction is performed by a Sim with the Socially Awkward trait.
If the mood level is low, they become angry instead of uncomfortable and furious instead of miserable.