I was shocked to learn that several common practices are actually not right according to merchant agreements with credit card companies. For instance, unsigned credit cards or those that say "See ID" are NOT VALID and do not have to be accepted. Some people would argue that they are more protected from potential fraud. I think it is worse to show a clerk my driver's license, but hey, that's me. Additionally, merchants that post signs such as "$10 minimum purchase required if using a credit card" violate user agreements. If you see a sign like this, you can report the business to the credit card companies. I do not know anyone that would take the time to actually do this since most places with those types of signs are smaller independent businesses that have to pay the credit card fees like anyone else. There are lots of grey areas here, but the important point is to be aware.
Check out this article for more:
10 things you might not know about your credit card from consumerist.com