Lack of coordination between the different departments within a business, must be the biggest barrier to communication. It affects both internal and external communication. Unfortunately, it's an issue at the business where I work.
The different departments at our business do not share information as often and as quickly as they need to. They either do not communicate on an issue and therefore have no idea what the other is doing, or they communicate but too late.
I think this has to do with management. When management doesn't have clear rules about how coordination ought to take place and who ought to speak to whom on a regular basis, there is confusion. And it also has to do with the personality of the employees. Employees who are not interested in coordinating their activities with others are a hindrance to communication themselves.