The site is built with the meteor framework, so it's not just the simple implementation you might expect from the layout. I imagine the features of meteor are useful in the actual app itself, which is probably not a simple site.

Honestly, I've been taken a bit by surprise that the back button bug has been such a big issue. I did some testing among friends (with a group of five or so startup, programmer types) and, believe it or not, it never came up.. That said, it'll be fixed in short order.

To the other comments, the service is considerably more complex (and yes, built on Meteor, MongoDB and ElasticSearch) as it serves up search results and updates in real time. Though the UI is simple, it does take a fair bit of effort to organize all the browsing information and index such that search results are relevant.