Ethan,
Without getting too deep into it, Sketchup doesn't follow the same interaction and interface metaphors as 'traditional' 3D programs. It is much more free-form than they are, and is more akin to drawing with a business presentation-type of drawing tool for basic shapes and then 'pushing' and 'pulling' those shapes to get into the 3D realm. You would understand that as extrusion, but that term—and many of the tools that are associated with that term in other CAD programs—doesn't really show up in the Sketchup interface. Similarly, you don't have to specify explicitly which axis you are drawing in, as the drawing surface is simultaneously live in all three axes.
It really is significantly easier than CAD for most people to learn. If they can make a Powerpoint slide of an org. chart, they have all the basic concepts they need in order to be productive. Of course, as with any program, once you immerse yourself deeply, as Dave has done, then there are all kinds of methods one can use to make one's work more elegant and efficient. But for just getting started, it has a significantly lower (and different) threshold of knowledge over CAD and 3D illustration (Maya, etc.) programs.
K