Arc from 3 points, add tangent/normal to a curve on a determined point, measure length of polylines and splines, measure area of polygons arcs and ellipses, measure angles, library of elements, all the 8 snapping modes and indication of what each mouse button will do.
Nice! So, I think the UI must make these features more obvious.
Another thing I miss in inkscape (in case you know): I sometimes did some very interesting interactive live geometric demonstrations with Kig. Had inkscape had the same features, I'd use it.