flatness

noun

A lack of excitement, liveliness, or interest: asepticism, blandness, colorlessness, drabness, dreariness, dryness, dullness, flavorlessness, insipidity, insipidness, jejuneness, lifelessness, sterileness, sterility, stodginess, vapidity, vapidness, weariness. See excite/bore/interest.

The noun has 2 meanings:

Meaning #1: the property of having two dimensions
  Synonyms: two-dimensionality, planeness

Meaning #2: the property of having little or no contrast; lacking highlights or gloss
  Synonyms: lusterlessness, lustrelessness, mat, matt, matte