So I was thinking...
Perhaps the draw mode ('fill' or 'line') of primitive shapes should be a state, like color, line width, etc.
Is there anything which makes it that much more suited to being an argument instead of a state than color/line width/line style is?
As well as being consistent with other states that aren't arguments, it would also be a bit more consistent with other things LÖVE can draw, such as lines and points. Lines and points only have coordinate parameters, instead of drawing mode and coordinates.
The interface could look like love.graphics.setDrawMode('fill'), or maybe even love.graphics.setFill(true).