“Tenaciousness” for “tenacity” (a physicist interwiewed on ABC News)
“Impetuousness” for “impetuosity” (from Inversions, by Iain Banks)
These seem to me to illustrate the two poles of “ness” monstrosity: “tenaciousness” is a long, ugly substitute for “tenacity,” a nice, vigorous word.
By contrast, “impetuosity” actually adds a syllable to “impetuousness.” The claim that “impetuosity” is superior must rest either on educated usage (since it’s hard to argue that the -ity ending is truly more regular than the -ness ending) or on the avoidance of the ugly repeated sibilant of “ousness.” [Update: my source points out that the stress pattern of “impetuosity” makes it more suitable in poetic us than “impetuousness. Fair enough.]