In the early days of Inkscape, njh changed the default style of shape tools to include fill-opacity:0.75. The motivation was "to make default shapes more interesting" and "to demonstrate that Inkscape can do opacity". Now, with a master opacity control in the statusbar, the second reason is much less relevant. Besides, we slightly discourage using fill-opacity and stroke-opacity in general, because they are somewhat more cumbersome to change - you can only do it in fill&stroke dialog. For these reasons, I have removed fill-opacity from the default style of the shape tools.