Ajax is a nice method of adding content to a site / page in such a way that the site is fast because only the content that is different needs to be downloaded to the browser.
HTML5 adds new power to partial page rendering methods. Including a client side database, which information can be pulled from. The one problem it has is this information is not seen by Google; It is unsearchable with Google. Being search-able is important for sites that get most of their visitors from search engines.
A site that uses AJAX for the majority of their content that depends of search engine traffic will have a very hard time. The only way for Google to know what the site is about is external links – without the external links the site would wither unless it has another method to bring in visitors.
Other methods do exist, Facebook, Youtube marketing, offline marketing … Adding a blog to the site to educate about the services can overcome this difficulty.