Know what I hate? Old-style numerals. Here's a picture to illustrate if you don't know what I'm talking about:
Besides the fact that I think they look ugly, I find them terribly unpleasant to read, though not necessarily harder to read. Even worse, they don't have one quality that I find very important in a good typeface: all of the numbers are exactly the same width. There are some particularly high-quality fonts, such as Adobe Garamond Pro in the example above, which include both styles of numbers, and I'm fine with that, of course.
I don't know why there exist modern, non-decorative fonts that have this feature. It's fine if you're Photoshopping a wanted poster and you want your text to look like the damned Old West, but they're still tacky.
Most of the typefaces on your computer probably have normal numerals. The only standard Windows font that comes to mind is Georgia. With Vista and Office 12, three more will be added, much to my chagrin: Constantia, Corbel, and Candara. It's too bad, because those three are all otherwise very nice fonts. All four of the Microsoft fonts that have this characteristic are designed specifically for readability, but in this case I think it comes at the cost of too much aesthetic appeal.
Do any of you actually prefer the old-style numerals to the normal ones? Don't be shy; I won't hate you if you do.