Users can now use the UI (user interface) to create more plug-in like scripts. This will save you enormous time when let's say you need to fetch a file/change variable without having to change your script from inside.

Users can now use the path tool it could come in quite handy if you wanted for example to dynamically create shapes to save them to your shape library.

You can use new CS functions such as the Gallery, and set all its options.

You can use layer 'bounds' to determine the width & height of the layer and therefore position it at will on your canvas (which was impossible to do in Photoshop 7)

You cannot use some simple tools such as the eyedrop color picker tool (a work around is to crop a 1x1px image, and use the histogram to fetch the color, but it is heavy coding for something quite simple)

You cannot script some Photoshop functionalities such as automatically generating your brushes/patterns/shapes or simply

You cannot script mask usage

You cannot add user interaction in your script (ie your script runs from a to z), it would be handy sometimes to be able to use the cursor position etc...

You cannot paint using scripting.