There are many things that can make a good idea. A good idea is one that solves a problem, creates something new or improves an existing solution. They are often unique and can in the best cases be expressed in simple understandable terms. A good idea is not just original but also simple and practical to implement. It should be well researched so the implementation process goes smoothly. A good idea often has a clear goal which makes it easier for others to understand what's being suggested. Ideas that are important can make a significant positive impact on society and can improve the lives of people. When trying to come up with good ideas, think about how it will impact people and our nature.
What separates a bad idea from a great idea is also it context and timing. Inventions such as the car, computer, and television were considered radical ideas when they first appeared because they were new but that did not make them bad ideas. Suggesting to invent the TV now would be be a little late - just as it would be hopeless to suggest drinking more water to solve thirst, if there is no water around.
In the end, the power of a good idea is dependent on the execution. A great idea in your head or on paper means much less if it is never carried out in the real world.