This one overlaps into the category of being on time, but in today's competitive environment, stopping to chitchat when your colleague is under the gun, or wasting time in a poorly planned meeting is also a business faux pas. Don't have poor listening skills. Being a good listener is key to being successful. Don't interrupt or hog the conversation. You can ask questions.
Good grooming is key to a professional impression.
If you are wondering if your skirt length is too short, then it probably is. And, guys, please leave the Mickey Mouse tie at home. Respect another person's space. In today's business environment, it's considered impolite to place your briefcase on a table or a desk. This includes using the phone without permission or sneaking a peek at the contents on the desk while waiting for your meeting to begin.