What is the most important feature of the update of html5 that is not present in previous versions?

Answer 1

It is not easy to choose the best one. I would say that for all of us it is pretty good that we have new input types (very useful and js validation code is shorter now).
There is also canvas so we can draw via JavaScript taking advantages. I have to include audio and video elements so as geolocation API and much more..
Can't forgot of session and local storage!

Answer 2

All new features are mostly important, the more depending on your field of application ^^