Though a professionally designed site can do wonders for credibility and enhance the user experience, if you’ve got great usability, users will come. From the article: “Let’s be honest though and admit that there are two kinds of simple designs that you can produce: Ugly [and] Beautiful. In either case if you are offering what the user wants they will use the site. It never was about the ugliness of a site, it was about its usability, community and a couple of other things.”
Think Ebay, Delicious, and Craigslist. I know I’m missing lots more, but you get the idea. Nail both usability and aesthetics and you’ll have a knock out like Flickr, though.