This question is simple and complex in nature. A good marketing site contains the necessary elements that the search engine spiders look for when they place information on top positions. The basic elements that need to be reckoned with are: a. Title b. Metatag Description c. Keywords d. Ad Content When you arrange these simple elements correctly, then, and only then, are you ready to begin the quest for a top position. Any qualified seo firm should be totally aware of these elements, and if they are not, you should take your business elsewhere. I have dealt with some of the top webdesigners in the country, and undoubtedly, most of them do not even consider these critical elements. They are more concerned with the look and feel of the site, and not with the essential ingredients found within that site.
perhaps your title is wrong!! your title says "What makes a good marketing website?" but your body talks mostly about SEO. Perhaps a good Mktg Site should contain at least 2 types of "marketing pages": a squeeze page - used to extract emails; a sales page - used to post your sales offer. Then you also have the "thank you" page, the "about" page, and a few others.