What makes a good trademark?

Those in the initial stages of developing branding for their businesses or rolling out new product or service names or slogans find themselves in the position of attempting to select a trademark that is both legally protectable and marketable.  Many of my clients with business experience are able to determine what makes a trademark strong from a marketing perspective.  However, many business people forget to take into account that what is strong from a branding perspective may not be the best choice of trademark for legally protecting your brand.