Not only has ActionScript changed, but the Flash CS3 authoring environment also sports an entirely new look.  The new interface sleeker and much more flexible than anything Flash has seen before.

This new interface allows you arrange and dock your Flash panels just about anywhere using drag and drop (those of you having used Flash 5 may reminisce). Additionally, panels can now be collapsed into icons which, when clicked, open up the panel as a fly out. 

This allows for maximum use of space while still retaining easy access to your much needed panels.  For example, you could iconize the Actions panel where ActionScript is written on the right side of the screen and have it open with a single click only when you need to write code.

When you write code in Flash using ActionScript 3, you’re writing code on the timeline and only the timeline.  If not writing in the timeline then you’re writing code in an external class file (at which point you wouldn’t be writing code in the FLA, now would you?).  In ActionScript 1 and ActionScript 2, you could write code on the timeline as well as on objects such as buttons or movie clips when adding code to the Actions editor with those objects selected.  Code then would be added within on() or onClipEvent() blocks and related to some kind of event like press or enterFrame. This is no longer possible in ActionScript 3.  Code can only be written on the timeline. This includes all events, like press and enterFrame as well.