Why does cel-shading sometimes look similar to rotoscoping?
I checked out the Wikipedia page for rotoscoping and it says one way it was done is that a live action image was shown behind frosted glass and an animator would draw it.  The frosted glass detail obfuscation coupled with the fact that it was a cartoonist means to me that you are going to get solid colored areas as shading, like you'd see in a cartoon.  Cell shading is basically trying to make the same effect but using 3d rendered graphics as a source instead of an actual photographed image.  Seems like they are the same effect, just with different names!