Poseidon was very violent. Sometime he liked to kill people just for fun! He wasn't always this way, but when he was, he was one of the most dangerous gods. This was one reason why he attacked Odysseus for blinding his son (Polyphemus the Cyclopes) even though the Cyclopes was threatening to eat Odysseus and his crew. He could also be very vengeful.
This was a bad trait because it meant much death for sailors and others who angered him.
Odysseus and Polyphemus
How do think sailors were supposed to cross the seas if Poseidon was always angry? When he was in a good mood, he would drive a chariot drawn by horses or hippocampi (half horse - half fish) and he would calm the seas. Sometimes he would even withdraw previous floods or water damage he had made.
This was one of his good traits because without it, no Greeks would ever be able to cross the seas, meaning no trade and no travel.
Poseidon in his Chariot