Is is actually tables? I haven't actually looked at the source, but I imagine that tearing out all the templates and styles, and redoing roughly the same design can't be more than a day or two's worth of work.

Maybe there's a fear that if it got too pretty, they might start to attract the wrong crowd?

HN hasn't always had an API, so they didn't want to change the HTML because it would break the apps that rely on scraping the markup to get the content. You can read more at this blog post [1] announcing the HN API.

[1]: http://blog.ycombinator.com/hacker-news-api