> I thought the entire definition of AO is that it's a rough approximation 
> to true GI that's useful because it renders really fast?

Indeed.  Raytracers typically cast out a few rays (the more the smoother the 
result) to see if they hit anything within a certain distance.  GPU methods 
typically just compare the depths of the surrounding pixels (so it's 
essentially done in 2D).  Obviously for a GPU this is much faster as it 
doesn't need to access the scene data (it might not even exist in most 
cases!).

> As far as I know, I'm not using any ray tracer features, and yet the final 
> render looks different than the edit window. (If only by being more 
> grainy...)

If it looks grainy then you've probably got AO turned on with a low number 
of rays.

Note that in the edit window there are several different options for the 
display mode (wireframe, shaded, textures etc).

> Well, I suppose. (I idea why AA would be better...)

AFAIK standard GPU rendering uses a very limited AA algorithm (usually fixed 
to a certain number of sub-pixels and some maximum like 8x), a raytracer 
usually can use dynamic AA depth and as many rays as are necessary.

> I guess I usually think of the main advantage of ray-tracing as being the 
> ability to render things that aren't triangles.

I think there are some raytracers that can *only* render triangles.  I guess 
it makes them simpler and they can be highly optimised for huge lists of 
triangles.  Then you can make some layer that converts higher order shapes 
to pixel sized triangles on the fly to feed the raytracer if needed.  IIRC 
this is how Pixar's raytracer works.

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