The most obvious new feature of Flash CS3 is its new interface. As mentioned before, the new look is overall a very positive experience that takes the already flexible and quite useful 'docking-and-panels' type of UI thing that Flash already had going, and extends it a little to make it even more space efficient and customizable. The toolbars can be reduced down in size quite a bit and I particularly like the fact that the Tools bar can be squeezed into a single column width block, which saves a bunch of horizontal space without really changing its utility at all.
The expanding/collapsing 'dock' is another prominent aspect of the new interface, and by and large it is also a positive experience, however I have found that having the Actions or Output panel accessible via the dock is a bad idea - those panels are best used nestled in with the stage or timeline (or on another screen if you have one), as the constant need to hide/show them can get tedious.
The configuration of the panels and dock is highly customizable and as before, Flash CS3 provides a way to save a 'Workspace' so that the UI can be changed to suit the requirements of many different types of project easily.
Still on the look and feel, one aspect of the UI that I wish were absent is the small fade-ins and out the whole interface does when the user switches to and from the application in the operating system. This seems a rather unnecessary piece of visual candy that can be frustrating. On my previous computer (A PowerBook G4 that otherwise ran the software perfectly well) this fade-in time was often a couple of seconds, which gets in the way terribly when you're trying to think fast. It would be nice if this feature was absent or at least controllable via preferences (although it should be noted that on my MacBook Pro this isn't really a problem anymore, but it's still kind of annoying).Second on the list of noticeable new features, and probably the most important, is the integration between Flash, Illustrator and Photoshop. Adobe has graciously provided two excellent import tools for bringing Photoshop and Illustrator files onto the stage or into the library. Features include maintaining positioning, layering (including layer names) and even adjusting the stage size to the imported documents size if you wish.