2. Too much contrast
Too much unity - no change in texture, key, mode, melodic and harmonic direction, too many repetitive rhythms.
Too much contrast - constant change of new textures, themes, rhythms, melodic contour, harmony, keys, modes, and other compositional elements.
A good improvisation (like a good composition) must have a balance between unity and contrast.
A remarkable improvisation happens when the listener (but not the organist) forgets about all these compositional elements and simply is in awe.