Nothing is more frightful than imagination without taste.

[Es ist nichts furchterlicher als Einbildungskraft ohne Geschmack.]

Johann Wolfgang von Goethe (1749–1832) German poet, statesman, scientist

Sprüche in Prosa: Maximen und Reflexionen [Proverbs in Prose: Maxims and Reflections] (1833) [tr. Saunders (1893), #489]
    (Source)

From Wilhelm Meister's Journeyman Years (1829). (Source (German)). Other translations:

There is nothing so horrible as imagination devoid of taste.
[tr. Rönnfeldt (1900)]

There is nothing more awful than imagination devoid of taste.
[tr. Stopp (1995), #507]

There is nothing more fearful than imagination without taste.
[E.g. (1868)]