Display & navigation:
* Smooth pixel rendering at any zoom level (via new OpenGL support)
* Bird's-Eye View for fast navigation of high-res documents; detailed explanation to follow
* Flick panning (aka "Hand-tossed pixels") for quicker navigation of high-res documents
* Support for multi-touch gestures on MacBook Air/MacBook Pro
Interface:
* Support for tabbed document viewing and quick N-up (2-up, 3-up, etc.) window reconfiguration
* Support for using miniaturized panel groups on secondary monitors. (The term "panel" now replaces "palette" throughout the Suite.)
* Ability to set the color of each screen mode separately (in a way that's now actually discoverable by humans)
* Ability to have no border around an image (useful for soft proofing in conjunction with custom background colors; call it the Stephen Johnson Feature)
* Flash panels enable a dramatically more configurable working environment, especially when used with the forthcoming Configurator utility app.
* Return to three "F" screen modes (vs. CS3's four)
* Application frame (optional on Mac for those who want more managed document/window handling)
* Improved automatic scrolling when zoomed in near image edges
Keyboard shortcuts:
* Spring-loaded keys: Switch from any tool to any other temporarily by holding down the other tool's key. For example, you can hold "R" to invoke the new Rotate View Tool, drag on the canvas to rotate, and release R to jump back to the Brush or whatever other tool you were using.
* Cmd-~ (tilde) is now assigned to switching among open documents, as is Ctrl-Tab, meaning Photoshop is now consistent with both Mac and Windows conventions
* Cmd-1 is now assigned to 100% zoom, as it is in Illustrator, Flash, and other apps
* Single-letter keyboard shortcuts for video, meaning you can navigate through time using just one hand while painting with the other
* Ability to delete layers via the Delete key when any tool is selected. In CS2 and CS3 you had to have the Move tool active, leading to the "V-delete" convention.
Color correction & compositing:
* Photoshop now emphasizes non-destructive techniques via a new Adjustments panel, making it faster and easier to browse & edit adjustments
* Curves, Hue/Saturation, and Black & White feature an on-canvas adjustment tool that enables you to click a region of the image, then drag to adjust just the targeted tonal/color range.
* Support for Vibrance adjustment (introduced previously in Camera Raw/Lightroom) for smarter saturation adjustments
* Improved Color Range selection tools
* Improved Dodge, Burn, and Sponge tools: Dodge & Burn do a better job of preserving tones while Sponge uses the Vibrance algorithm
* Ability to adjust the density (opacity) of a layer mask
* Ability to add non-destructive feathering to a layer mask
* CUDO support: Photoshop's soft proofing features can simulate color blindness, making it easier to create accessible signage and other artwork. (Illustrator offers the same functionality.)
Metadata:
* Better copyright preservation controls through Save for Web
* Unique IDs generated/stored per document (to enable easier asset tracking downstream)
* Layer-based metadata: PSD files now include timestamps that indicate when each layer was edited (thanks to the Pixar guys for suggesting this), and scripts/Flash panels can read/write metadata on a per-layer basis; more on what that means soon.
* Extensible, Flash-based File Info (enabling developers to add network-stored keywords, spellchecking, etc.); a cool in-house demo lets Photoshop and Bridge display Google/Yahoo/Microsoft maps for geotagged images.
Painting & retouching
* Smoother painting performance
* Drag-resizing cursors: Hold down modifier keys to resize a brush and/or change its hardness by clicking and dragging
* Live under-cursor preview for cloning/healing (a la Vanishing Point)
* Pixel grid view: At zoom levels greater than 500%, Photoshop will optionally display a faint grid that makes it easier to edit pixels precisely.
* Eyedropper option to sample all layers or just the current layer
* Flash panels will enable the creation of non-modal (panel-based) color pickers
Smart Object enhancements:
* Layer masks can now stay linked to Smart Objects while moving/transforming the objects. Sorry that it took a while.
* Perspective transform: You can now apply non-uniform transforms to pixel-based SOs (non-destructively, of course).
Filters/Plug-Ins:
* Adobe Pixel Bender plug-in for fast filters, enabling more modern versions of operations like Radial Blur
Miscellaneous:
* Ability to delete multiple channels at once. (Not paths this time; sorry.)
* Stroke color now defaults to black (hey, don't ask me, but people said they'd like it changed)
* Kuler panel enables creation of color harmonies, download/upload between Photoshop and kuler.adobe.com.
* Automatic detection & correction of vignetting & fisheye distortion during panorama creation
* "Share My Screen" command under file menu enables quick, free screen sharing with up to two other participants (same in other CS4 apps; great for quick reviews with clients, art directors, etc.)
* The Pen tool now defaults to drawing paths (instead of filled shape layers), while the various shape tools continue to default to drawing filled shapes
* Support for brushes up to 1500px in Liquify (up from a previous limit of 600px)
* Support for sampling color from fill layers (solid color, gradient, pattern)
Printing & color management:
* 16-bit printing (Mac only; requires OS X 10.5)
* Support for printing larger documents (>30,000 pixels)
* More automated print output enabled via increased scripting support
* Support for Device Link color profiles
* Support for abstract color profiles
* Support for N-color/multichannel imaging
* Support for scene-referred color profiles (useful with After Effects CS4)
