First off, the user interface is WAY better. It looks way nicer, everything moves around easier, it auto adjusts the sidebars when you change something, such as when you take out the color picker window on the right tool bar, it auto matically resizes the rest. Also another small but useful I noticed was when you put an image in full screen it centers the image between your two side toolbars rather than the middle of the window (which on big images like layouts wont let you see the whole thing at once, which was really annoying to me on CS2). Also another useful thing is smart filters, which basically allow you to apply filters to a layer then have them go on the layer toolbar right under the layer you are in and lets you easly edit them so you dont have to apply a filter then delete it and apply it again to fix small modifications to it. You can also set filters so they are invisible like on layers.
