What makes a great logo?

Every once in awhile, I get a new client who requests to have a rough mock up using clip art or a sketch they did, turned into a logo for their company. Some can be a great start and others, not so much. When I get a "not so much", I send them to this article on a blog I found What is a great logo? It explains what types of designs are effective and the principals that make a great logo.