There are several factors that one can consider in answering this question. First, perhaps the best place to start is to state that the idea of a "good design" is in the eye of the beholder. So, what is "good" may not be good to another person. Second, what most people will agree upon is that good design has form and content. In other words, the form and content must both match what is trying to be conveyed. So, from this basic starting point, there are three key elements.
First, what is the design for? Is it for a company, a newsletter, an event? You need to know these details well, so that your design will accurately convey what it is supposed to.
Second, what is on the design is important. Does this content match the purpose of the design? Too often the content is not clear and so those who see the design are left wondering.
Third, you will need to think about the form of the design. The form is a conveyor of meaning, just as much as the content.