One of the worst types of post is the one where people get mad at others for having a good time. This is really shallow! Are you mad because people are having fun but your life sucks? Then do something about your life to make it stop sucking! Once again, we all go through periods when we're down and life seems to be crapping on us. Those will hopefully be counteracted with moments of pleasure and joy. To tear someone down during those times is truly a hurtful thing.
Another type of post I hate is the totally vague post. "Oh! I'm so mad!" is not a good post. Why are you mad? Are you simply looking for attention and/or validation? If you're in a silly mood and just type something like "Porkchop Sandwich!" just to see what sort of comments you get, that's one thing. "Fuck it!" is another thing. If you want sympathy or an ear to listen to you, please be a little more specific so that we can give you the help/advice you might need.
Chat is another potential source of drama. If someone makes a off-handed insult to you on chat, do not throw gasoline on burning bridges. I have had a few arguments with friends because of something said on chat because an emoticon was left out. It is an imperfect medium of communication. You miss out on emotions behind the words that would not happen if you were talking FTF or even on the phone.
LJ is a place to put down your emotions as well as the day-to-day goings on of your life. Don't censor yourself or try to be someone you're not. That sort of defeats the purpose. But please! Think before you post. Are you spamming your friends? Are you putting down the same thing day after day? Are you saying something worthwhile?