How do you actually rate ideas to be good or bad?

If the implementation of an idea fails can the idea still be rated as good?

To me it seems that excellent ideas usually are rated excellent only afterwards, when they are already successfully implemented. The same ideas were just good or average before the implementation.

What is the primary function of an idea? Let's say it is to inspire and to show a direction. This definition would at least explain some of the above, wouldn't it?

Antti