Layers are certainly not a new feature in illustration software, but Adobe has done some really interesting things with them. One of the neatest is that a thumbnail is displayed in the Layers palette for each layer. This is very similar to the Layers palette in Photoshop. While this is a small change in the user interface, knowing exactly what is on a layer is so much easier by seeing the thumbnail. At left is an example of the Layers palette complete with thumbnails.
Transparency is another feature that comes late in the game to Illustrator.
I find the Interactive Transparency tool in CorelDRAW much easier to use, but Illustrator does have some neat tricks up its sleeve. Obviously you can apply transparency to objects. But the big benefit is that it can be applied to groups or layers just as easily. Another plus is that transparency can be applied to spot colors so that they remain spot colors when printed as separations. I must say that filling an object with spot colors is incredibly difficult as you must manually create each spot color you want.