Dissection of a Digital Illustration

What's the difference between 3D digital illustration and 2D illustration?

I think that, really, 3D digital is more similar to non-digital 3D work than to 2D digital. Painting in Photoshop requires more or less the same abilities as traditional painting. But to do a 3D project, you have to wear a lot of different hats, and learn new skills if you don't have them. You get to be a sculptor, painter, exhibit designer, and photographer. You sculpt your models, texture and paint them, arrange them in a scene with a background set, light the scene, and take a picture of it. And then touch up your photo.
The pros of 3D is that it is extremely powerful and flexible.
On the other hand, it tends to be a more convoluted and less straightforward – and sometimes more time consuming – process, than painting directly in 2D.