As Glenn mentioned, there are valid points to both sides of the argument. I personally think the stock classic green template that comes with the basic install is hideous. As a consumer who does most of his shopping online, I can tell you that - in the days since Web 2.0, HTML5, etc. - the older, basic looking sites I see look very dated, amateurish, and unprofessional at best, which is why the average cart owner scrambles for a template as soon as the installation is complete.
Maybe I shouldn't judge a book by its cover, but to me, it's a direct reflection on the business and how it may (or may not) operate. Probably also why Prestashop has become so popular in such a short period of time - a modern-looking site can be had out of the box without having to learn to code in addition to trying to get a business up and running.
This is really no different than operating a brick and mortar store. Window dressing counts.
Having said all of that, when I DO shop, I expect my checkout experience to work as expected. A lot of research has been put into shopping behaviors and other ways to chase conversion rates, but at the end of the day, I don't care about personas, social tie-ins, etc. I just want my stuff at a fair price, and fulfilled in a timely manner.
Interesting discussion...