For me, What makes a good design is a difficult question, because it is subjective. It is a matter of personal taste, so it's hard seperating good design from bad design .But I would say A good design is meeting a given requirement, meaning if a client comes to you and ask for a specific design, and you make it exactly how they wanted it, it's a good design.
You should also consider if working with colour, because some people have emotional associations with certain colours, the same with shape and size. Design needs to fit a purpose, it needs to attract people.
Quality,functionality, uniqueness, I would say is a few words that you need in a good design, things that make a design good.
