Along with a slew of tweaks and bug fixes, the most notable new feature is HTML5 “pushState” support, which you can see in action by trying a search in DocumentCloud’s public archive. This enables the use of true URLs, but also requires you to do a bit of extra work on the back end to be sure that your application is capable of serving these pages, so it’s strictly on an opt-in basis.
Of course, not all browsers currently in popular use (ahem, Internet Explorer) support the “pushState” function yet. Older browsers will continue to use hash-based URLs, and if hash-based links are shared with modern browsers, they’ll be transparently upgraded to the “pushState” version of the URL.
Other changes include renaming Controller to Router for clarity, the refresh function to reset, and replacing saveLocation with a more flexible navigate API. There are instructions for upgrading from 0.3.3 to 0.5.0 that should help with these.
The full change log is also available.