Kidner's commentaries are a consistent go-to source for me. In his Proverbs commentaries he writes about the power and weakness of words, and words at their best. Here is what he had to say about the marks of good words (full references added to allow you to take advantage of mouse-over feature on this site):
1. They will be honest. 'Righteous lips are the delight of kings' (Prov 16:13) - one of the good things which the great can neither buy nor afford to be without. cf. Prov 24:24-26, where a 'right' answers (Prov 24:26) is literally a 'straight' or 'straightforward one. cf. also Prov 25:12; 27:5,6; 28:23.
2. They will be few. This point is made with iron in Prov 17:28 ('Even a fool may pass for wise, if he says nothing', Moffatt); but there are solid enough grounds for it elsewhere. In one's own interest, the less said, the less ammunition there is for ill-wishers (Prov 10:14; 13:3); for one's neighbour's good, reticence may save a friendship (Prov 11:12,13); and in relation to God, when words run away with a man they run him into folly and arrogance: 'When words are many, transgression is not lacking' (Prov 10:19).
3. They will be calm. A link with the previous paragraph is found in Prov 17:27, where a sparing use of words is commended as the mark of 'a cool spirit', which denotes 'a man of understanding'. Three reasons can be found for this praise of calmness: first, it allows time for a fair hearing (Prov 18:13; cf. verse 17); second, it allows tempers to cool (Prov 15:1 : 'a soft answer...'); and third, its influence is potent: 'a soft tongue breaketh the bone' (Prov 25:25).
4. They will be apt. A truth that makes no impression as a generalization may be indelibly fixed in the mind when it is matched to its occasion and shaped to its task. There is a craftsman's as well as a recipient's delight glimpsed in Prov 15:23 : 'To make an apt answer is a joy to a man, and a word in season, how good it is!' The same aesthetic pleasure glows in the language of 10:20 ('The tongue of the righteous is as choice silver') and Prov 25:11 ('like apples of gold in a setting of silver'); cf also Prov 25:12; 22:11; 10:32.