There is no list published by the US government as mentioned in other answers, but I know people who were refused US visas in different US embassies/consulates because they have visited one or more of these countries:
The list could be longer, and visiting one of these countries doesn't mean you will be refused; but, it matters, because they have a field in the visa application about the countries visited in the past 10 years. The guys I know were asked questions such as "what's the purpose of visiting". The rejection/approval totally depends on the interviewer and/or your answer. I have noticed the younger you are the higher chance of getting refused, if you visited one of these countries. I guess they believe the younger you are the easier your brain to be washed with anti-US stuff. Also I have noticed people who go to the US on regular basis they do not get refused! Usually these people have family ties. The last thing I have noticed is people who are from those other countries might have better chances of getting the visas while people who visited those countries might not get it! I also think the same logic applies for people who need electronic visas, where no interview is required, except from the customs guys upon arrival.
Anyway, as long as you already obtained the visa, then the chances of being asked again at the immigration/customs are low.
BTW, I work for a multinational company, many of the employees are required to have a US visa for business trips (hundreds) and that's my source. Also, I have been to all countries in the aforementioned list and I was asked the reason behind my visits but I always was granted the visa.