With respect to Liz, I have to argue the point that wikis are beautiful, not ugly. Their beauty is in not trying to be pretty. They emphasize function over form over aesthetics.
Joi Ito had some great comments on the difference between looking and reading.
...McLuhan talks a lot about how "looking" at TV is different from "reading" text. When you read a book, your eyes are focused a bit above the text and the text sort of just goes into your head to create symbols. With TV, you actually LOOK. You really care if the font on the TV is ugly, but you rarely remember the font of a good book you just read.
So, maybe this is the difference. When I am on a Wiki, the way it looks really doesn't concern me as much as trying imagine and understand all of the context that is captured in the web of pages linking to and from the page. I imaging all of the people from all kinds of places and what they must be thinking. It's less about user interface and more about code...
Its actually more about the text. Context is gained from the page's revisions, links and how it is referenced by and navigated from other pages. Wikis excel at logical context whereas blog excel at temporal context.
Blogs, emphasize form over function over aesthetics. The form of posts in reverse chronological order and blogrolls constricts possible uses and design. Sure many a blogger has tweaked their template and design to achieve superior ascetics, but they are bound to constraints that if surpassed looses recognition as a blog.
Wiki's aren't pretty, but that's the point. Except in rare instances where design creates function, the more you design the more user functionality you sacrifice.
Wikis emphasize both reading and writing. Sure they could be a little more readable, but that would come at a cost for writing. Costs to be carefully considered for a tool that enables a writable web.