Some things I have run across which make people uncomfortable, even when they're good acquaintances...

• Hugging somebody you don't know very well
• Trying to identify people based on their race. That political correctness makes us scared to admit that different races even exist sometimes!
• Maintaining eye contact for longer than a minute
• Being a close-talker
• Public Displays of Affection
• Depression
• Of course, anything and everything relating to masturbation. Sex isn't so much of a taboo anymore