Usable websites are all alike, every unusable site is unusable in its own way.
True, the stakes are lower here: no matter how broken your site may be, it's unlikely to destroy marriages or cause anyone to throw herself under a train.
Still, the tip of the hat seems appropriate: Unusable websites frustrate visitors in myriad ways, while the best websites have some fundamental things in common.
Usable websites have external consistency: Their designers understand that users spend most of their time on other websites, so they make visitors feel at home with conventions that feel familiar. Usable websites also have internal consistency: they are learnable, maintaining consistent navigation and look and feel from page to page.
Usable websites rely on underlying design patterns. Their creators recognize that “pattern” does not equal “clone” or “cookie-cutter." A well-executed pattern is a platform that lets your innovative product or unique offer shine.
Do you have an idea for the latest and greatest configurator for the widgets you sell online? Ignore the relevant patterns (forms, multi-step processes, results pages) and you may find a new way to make users unhappy. Pay attention to the familiar and sell something new.