For Craigslist the function is most important and credit should go to the back end guys. Something like Twitter would give it to the front end guys. I'm leaving scaling out of the equation for now and assuming all sites will perform the same under any load.
A site that is ugly doesn't get used. People always judge a book by its cover especially these days. But if the pretty site doesn't work then you're also screwed. I really hate this debate over who is more important: front or back end. You need both. Period.
I also hate the whole "design is easy" / "no, programming is easy" argument. Neither one is easy. They're different animals. The way you approach the front end and back end are totally different. I'm a generalist but lean toward design. I'm in awe of the back end guys but then some are in awe of me. We have totally different goals in mind when working. The back end guys are concerned with functionality. Security, scaling. The front end is all about beauty, load times, SEO, user experience. At one point in our work we do end up in the middle. That middle is when we're both thinking about the the front end is interacting with the back end and how will we code everything so that A) we can easily connect the two and B) we can efficiently extend the front end to accommodate new back end features and vice versa.
I understand the article wasn't exactly pitting programmers against designers but there's always that subtext and people always start thinking about it. It's a shame that the designers haven't gotten as much credit until recently but at the same time, even as a designer, I must give huge props to the back end guys as I understand the pains they go through.
But can we put the whole front vs. back debate to rest already? You just can't have one without the other. Period.