Before we get started I just want to tell you that nobody here is going to slap you on the wrist if you do any of this stuff, but we’re watching you – lol. All gamers pick up bad habits along the way; some of these habits are even considered taboo. But it's not really anything that would land you in the slammer – it's stuff that just annoys the heck out of other players at times, and can get you kicked off servers if you're not careful. So before you go ahead like a proverbial bull in a china shop, think about what you're about to do because some things really are taboo!
MMO is all about interaction with other players. In this vein, you have to be respectful and mindful of other players, even if you do so in a jocular fashion. There are ways for you to insult people without getting banned from the game. Sarcasm is totally okay, and gamers use it all the time. But don't curse somebody's mother, for Pete's sake. Once you start interacting with other players, you will learn how people communicate with one another. This is especially true if you are a noob. New players simply have to learn the ins and outs of how things get done.
If you're in a guild or group, and there are rules you should follow those rules. Breaking those rules is considered taboo and might get you kicked out of the group. Some of the rules are crazy – but being a part of the group means that you are an adherent of those rules. For example, there may be a pecking order when you are in a raid and when you’re collecting the spoils of victory. You don't want to take anything that you don't need, or something that is not supposed to be allocated to you given what your duties are in the MMO. Different games have different drops that players can collect, and sometimes it is common courtesy to share your resources with members of your group who need them. Not doing so is kind of taboo.
The last thing you want is to be blacklisted from the server for persistently bad conduct. That type of behavior is taboo and is likely to get you kicked off the servers of many different types of MMO games if you persist in that way.
Some people consider changing your class in MMO games is taboo. It's totally okay nowadays to change your character, change your class and even switch sides and play different characters if you want to. If you play to the end of the game with one specific type of character or class of character, it should be okay to switch characters and level up to that same point with a different class. Some people however still consider this to be taboo, although I tend to think of it as okay. But remember that changing your class is not the same as changing your role in a specific class.