Subtlety and lack of contrast. The old style fonts have heavy lines and light lines but the contrast in weight is subtle, not stark. The moderns are the opposite of old style fonts. These fonts typically have more character, and more attitude than their old style counterparts, and can be used to add character to a document rather than to typeset a long piece. They are based on the designs popular in the 19th century and later.
Vertical lines are heavy, horizontal lines are light.
Many moderns have a stark contrast between light and heavy strokes. Modern typefaces with high contrast between light and heavy strokes are not as readable as the old style fonts. Bodoni is the most notable modern. Transitional Transitional fonts fit somewhere in between moderns and old style fonts. Many of the transitionals have the same kind of readability as the old styles.