There are probably 5 most talked about features
- Web workers - allows developers to run scripts in background work so a web application can do more than one thing at a time.
- Video tag - An easy way to do flash free video online, we just need a standard codec
- Canvas - Exactly what it sounds like, a way to create an image on the fly within the browser window
- Application Caches - Allows applications to store stuff so that you can use them later without being connected to the internet (Read your email later after it gets downloaded, Like Google Gears was)
- Geolocation - Ability for the browser to relay it's location to the application
Many of these features have already been implemented in some browsers and applications. Google latitude uses geo location, Google gears was a plug-in based implementation of the same ideas as application caches, Canvas like things have been done with JavaScript.
Web workers is new and can allow for a bunch of different things to happen inside web applications. Any kind of side processing that seems to work now, will be more smooth as a single thread can be dedicated to notifications or an in-site chat or other periodic processing task within the application.
Resources:
http://geektechnica.com/2009/06/5-amazing-html5-features-to-look-forward-to/