The central criterion of excellent metaphor is the quality of the underlying comparison. Its resemblance must fit the intended subject and borrowed image, advance the speaker's purpose, and remain intelligible to the audience. The connection may be directly clear, commonly understood, or recognized within a specialist convention.
In realized and representational metaphor, avoiding overt simile preserves the force of the metaphorical claim. Where two ideas are strongly associated in one respect, direct metaphor may offer greater emphasis and economy than explicit comparison.
Implicit metaphor depends on the fitness of its concealed likeness, while imaginative metaphor depends on the quality of that underlying image. Successful metaphor thus requires more than unusual wording: it needs attention to meaning, relation, and audience.