What usability problems?

Consider that most painful of topics for web application developers: the back button. Web developers armed with some sample code, a decent DOM reference, and a lot of perseverance can build a pretty decent dynamic UI in modern browsers. These UIs doesn't jarringly destroy the user's in-page experience for the most trivial of tasks, like adding an item to a list. When larger portions of an application are mediated in this way, the user naturally has more desire to "go back" to some earlier state if things aren't working out the way they had planned or if the action isn't what they expected. An example might be switching between a view and edit mode in a content editing application.

As high-gloss web applications become the norm, many interactions become intra-page and not inter-page. Programmers looking for creative solutions have chosen XMLHTTP for these scenarios, but unfortunately, XMLHTTP breaks the back button, impairing the user experience. If the back button doesn't function in a way that meets user expectations, it becomes ever easier for the user to lose work or become confused about the state of an application. To assist the user, programmer need a way to capture back-button presses and do something intelligent with them.

If you've used Google Maps and you've tried to send your directions to a friend, you know that not being able to simply copy the URL out of the address bar is significantly confusing at first. Applications that dynamically construct large sections of the UI (like Google Maps) today resort to a link in an intermediate screen that the user can click to return to their current state and then, perhaps, book mark or send to someone else. And this is if and when they consider the "bookmarkability" problem at all. More common is an application that simply refuses to acknowledge that the user might want to pass around a URL to a friend and instead builds some heavyweight and non-standard state serialization mechanism that is more akin to a desktop application's "save as" feature. "Save-as" on the web is book-marking, and usable applications recognize this (even if they don't have great solutions for it today). Regardless of what serialization mechanism is in use, being able to represent the state of the application in a URL (or a marker for serialized state) is a must. This is a hard problem to be sure, and none of the currently available tools provide simple answers.