Hinting--this can make or break any screen font.
Simple font shapes are easier to read. Slab serif, and sans serif faces tend to be easiest to read. Most people find
serif faces more familiar--even on-screen. So sans serif type isn't necessarily most comfortable for people to read.
Larger x-heights help, because they give each letter more pixels to work with. Too high of an x-height, however, can make it difficult to distinguish between upper and lower case, and can make lines look crowded without extra leading.
Character spacing is crucial--fonts
that are too tight cause letters to touch on-screen. Touching letters make on-screen reading much harder. Faces with a little extra space between the characters are much easier to read on-screen.