Black-on-white is my usual preference, but white-on-black works decently with a sans-serif. With sans-serifs, a greater leading is more comfortable (1½–2). With serifs, the same goes, but also a normal “book” leading is fine (1–1¼), too. Size doesn’t bother me usually. Anything from 10–16 pixels is perfectly fine. What does bother me, however, is how wide the text spans. A span of 30–40 characters (or multiples of the size) across is most ideal. Anything less or more will bother me more than anything else (which is why I’m bothered by this forum’s new fluid-width design).

The link Kroc posted uses a large serif font, which is fine, but the accursed shadowing bothers me. I like the appearance of inset text effects when in short bursts but long-running text, however, doesn’t benefit IMO.