Good Idea

Good ideas are vulnerable. 

Vulnerable to attack from ourselves, attack from outsiders. It is commonly not accepted because people hasn't seen it before.

Bad idea is commonly accepted. As it has precedent, has been commonly done and can be commonly seen.

What is good idea?

To define it, looking at successful new idea examples don't work. As the time when you find one as an example, it is already an established idea. 

Established idea also means it has worked and commonly accepted by a particular group, if not, all the people. This makes good and bad ideas hard to be distinguished.

Since it is already accepted by people, you won't feel that it is of any unacceptable either.

Good idea is new. 

Which means it is even harder to conceive. You cannot simply ask: "What do you think about this idea?" to random people with no great judgement system. If people hasn't seen anything like it, the feedback you get is usually negative.

Good idea feels weird at the beginning, unacceptable at first, celebrated later on.