An interesting question and the answer depends on the culture of the company you work at and the industry they work in.
-Makes it difficult to understand what you are saying. Can be distracting. Conveys an “I don’t give a shit” attitude.
-Rude and can be offensive
*Wearing jeans and t-shirts
*Listening to heavy metal or rap, instead of elevator music
-Loud and pounding music, often with violent imagery is distracting and potentially offensice. Elevator music or “Muzak” is, if not enjoyable, is at least soft and unobtrusive
*Talking about sex
-Also potentially offensive
*Eating during meetings
-See “chewing gum”. Also you are focused on your ham on rye, not on what is being said.
*Having pages ripped from Maxim taped on your cubicle or office walls
-Distracting to others and offensive
*Having a GI Joe or Tweety Bird collection
-Gives an appearance of immaturity
*being blunt and aggressive
-Because you may convey your message but people won’t want to work with you again. Passiveness is not encouraged either. What is expected is to maturely and assertively state your oppinion.
Keep in mind that in many of these cases, while it isn’t affecting your job performance, it might be affecting other peoples.
As you said, you deal with this in school. School is for children, the workplace is for adults. I deal with clients daily and they want to know that when they are paying a lot of money for our services, they are paying a a competent, mature professional. Unlike a classroom environment, they might not know me from Adam. I need to build trust right away and it’s hard to do that while eating a sandwhich in jeans and a T-shirt.
That said, there is a certain amount of theatrics to “being professional”. We swear and make sex jokes and eat around the office all the time. My manager and I will bust out Simpson or Family Guy jokes or whatever. But we all know each other and have built up that trust. We can even be a little more relaxed with clients who weve worked with for awaile and know and trust us. But when we are working with the clients or outside vendors, it’s time to put on your “game face”.
The other issue is that unprofessionalism can become contageous. If an environment is too relaxed, sometimes it does affect performance. People spend more time joking than working. The start to get a sense that “anything goes” and next thing you know, half the office shows up two hours late to work.
Maturity isn’t about having a stick up your ass. It’s knowing when it’s time to be serious and when it’s time to have fun.