Good design is intuitive, creative and intelligent. The best design should not be so self-aware as to intrude on the message it is attempting to convey. Viewers should be moved to respond in the intended way by the message, supported by the elegance of simplicity.
Over-design can crowd your message, making intention and communication less clear. The same goes for content… for example in infographs. It is important to think about what you want your audience to take away from the design, let your designer know your key messages, and make sure they know the priority message. Keep them clear and simple.