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
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.