Yes and no. It really depends on what you're trying to achieve.
If all you want is some posterization/canvas style on the mesh surface, + extend a basic outline, then yes, you can do that!

However, you can't do multi-pass effects, and you can't do post effects, which many cartoon style shaders want.

(Closing this, as it isn't really an idea. SF2 may have multi-pass support)

While 0 isn't default, it's now possible, as of Beta 0.37, to write to the alpha channel but keep an opaque blending mode, so I'll mark this as finished :)

This has now been fixed in Beta 0.37

A Value of 1 should work, but try 0 as well

Glad you like SF!
While I don't think the node itself will support more inputs than the current one, you could simply stack multiple lerp nodes, one after the other, as seen here:
http://acegikmo.com/shaderforge/wiki/index.php?title=Texture_Splatting

You're not supposed to use the distance node in this case, use the Dist input of the Panner node:
http://www.acegikmo.com/shaderforge/nodes/?search=Panner

I've never actually looked into doing proper particle shaders. What's the main difference? You do the model/view matrix transformation, and then not do the projection matrix transformation until after the vertex offset?

A water plane that writes depth sounds, odd. It basically means everything behind it would fail the depth test, and thus not render

A week or two, most likely

As do I, but I got comfortable with it after about 5 minutes; I think you will as well :)
I'll probably force it to only be in one direction for consistency among users, especially those making tutorials/screenshots.