With the default Wiki CSS, list items are rendering at a font size progressively smaller than the surrounding text (at least with Firefox 2.0). The 2nd level does not read very well, and the 3rd level is unreadable.
This can be seen very cleary in the "Preview" button of the "Edit Stylesheet" page.
I think that as a default behavior, having lists items rendered with equal font sizes far more suitable.
Actually, the problem appears to be, not in the default CSS shown to the user on that page, but in some other CSS loaded elsewhere which is also affecting the Wiki.