When I go to Type Font I used to get a flyout that showed the preview of the font list. I don't get that anymore and can't seem to find a way to get it to return. I am attahcing a copy of my type page from my preferences file as well. So what I am doing right now if I just want to look for a particular style of font I am flipping over to Photoshop and view the list and then go back to Illustrator to select the font that I decided on.
Have you tried unchecking the glyph protection option in your prefs? That seems to be the cause of a lot of font menu weird behaviors. If once it's unchecked, your previews come back, it might be an indication of a corrupt font.
If that doesn't work, I'd delete your preferences.
As Jesse said, the evil glyph protection at it again, I'd say. On a hunch, you could also preemptively delete any fonts you may have added just recently.