As many experts as possible get involved in the development of a new website - marketers, seo experts, writers, developers, business owners. There's a team of people involved, often with very fixed views and ideas as to how they want the site to perform and how best to achieve that. Usability testing (which can be done at the very early design phase) is a great way to get a view from people that really count - the users. Learn how your site works in the real world!