Question: While reading this wikipedia [article](http://en.wikipedia.org/wiki/No_Silver_Bullet), Brookes has told that there is a difference between "good" designers and "great" designers. What is the difference between them? How can I decide if a designer is good or great?

Answer: 
I'm going to assume you're talking about Software Design as opposed to the narrower Ux design.

I'll quote a semi-famous passage from Alan Perlis:

"Fools ignore complexity. Pragmatists suffer it. Some can avoid it. Geniuses remove it."

One aspect of a Great designer is that they reduce complexity.
A good designer creates something that does the job well. A great designer also makes the user say "wow!!!".

A good designer knows the established standards, and applies them well. A great designer goes beyond that and creates new solutions that become new standards.