I searched this site as well as the "300 new features" on Apple.com before submitting and couldn't find this in either place. It seems kind of a "DUH!" feature, but still blew me away as one of the handiest things ever, especially for designers and font freaks.

In column view, select any font file (I tested TTF, OpenType, and both screen and printer Postscript fonts), and flip down the preview triangle to see a preview of the font in Uppercase A-Z, Lowercase A-Z, and numbers. In icon view, list view, and the small icon of column view, the font icon shows "Ag" in the corresponding font (I guess that means this is a 'Silver' medal hint. Hehe.)

Cover Flow view flips through larger "Ag" previews, and Quick Look shows a big Alphabet preview. Thanks Apple! It's the little touches like these that make me a 20-year Mac fan.

[robg adds: As expected, you can also see this in the preview icon in the Inspector/Get Info windows.]