Validation is the main area of improvement. HTML5 now provides a number of new validation features such as mandatory checking, type checking, range and field length validation. The validation is done within the browser, and developers can opt to decide how to process errors.
Obviously validation has to be repeated on the server for security, to ensure that data hasn’t been hacked in the browser or in transmission. This then means that validation has to be maintained in two places and kept in sync.
HTML5 also provides a number of new input field types such as tel, email, color, datetime. This empowers the browser, by applying it to display a date picker, or a colour chooser for example. More importantly for mobile applications it would allow the browser to show an appropriate keyboard layout e.g. a numeric layout for tel, and an alphabetic keyboard for email type.
There will be some organisations that will not want the browser to affect their carefully designed user experience; for these people the answer is simple, just don’t use the new features.