I'm going to take single page websites meaning a site that has one consistent page with AJAX loading for inner pages or content. There is a trend going on in making landing pages as one page containing a summary, but the purpose of that is self explanatory.
Generally SEO sucks for single page websites as a crawler wouldn't index it properly (or so I was lead to believe). There are however some workarounds for this that involve a "pseudo site" that forces a crawler to crawl indexing those pages with URLs that will lead to a page being loaded in your "one page" website. That was not very clear.
As for usability, a single page website would work well if there aren't many "inner pages". A simple web application would be an example.
Not sure about your other criteria.
In your example are you referring to the landing page or the actual web app?