It’s an interesting paradox that some poor choices as trademarks can be closely related to excellent marks. Probably the #1 reason we give negative opinions on trademark searches, and the #1 reason for trademark rejections by the trademark office, is that the proposed mark is descriptive.
A descriptive mark, or as the trademark office likes to call them, marks that are “merely” descriptive, are marks that “merely” describe the goods and services. Because they are descriptors of the kind of the goods, they can’t serve as identifiers of the source of the goods. Therefore, they can’t act as trademarks or service marks, because the purpose of marks is to assure the public of the provider of the goods, not the nature of the goods themselves.