Website in order to attract heavy targeted traffic needs to be user friendly and have a gimmicking look and on one hand and search engine friendly on other. User friendliness helps to increase the number of your returning visitors developing users trust and loyalty towards you. The other end of the discussion ie search engine friendliness means that your site must contain food for search engine spiders, bots and crawlers etc.
Bringing the both ends of friendliness together is a little complex, but none too difficult. Good know how of tastes, choices, preferences and interests of different age groups and genders help identifying the sort of design your visitors can be attracted to. For instance teens like bright colors, cool colors attract the attention of people above, almost all age groups love smooth and simple navigation etc. And knowledge of strategies search engines use to crawl different pages index and rank them etc can help building a site in a way that attains good amount of organic traffic. For instance, search engine spider doesn't see a page as a human visitor does instead it reads from source code; it doesn't read java script and frames, loves text content, doesn't like too many links etc.
It reveals that good design alone doesn't ensure the success instead it works well in combination with good code. A site with good design and not so good code can have difficulty in becoming a search engine friendly site. This fact requires incorporation of structural website design. There can be other factors determining the success of a website design, ie niche, natures of website, complexity of the software or systems it's using and above all the budget, and they all vary, but the user friendliness and search engine friendliness are the requirements of almost all type of websites, so are of utmost importance. And thus one should make sure that all these ends are given proper attention, and in case if you're hiring a freelance designer, do discuss the structural elements of website designing with him.