Aspect: "Auto" does nothing except maintain aspect ratio, and allows large black borders (right, left, top, bottom) on some non-HD streams (those being rendered in 16:9 format within the standard non-HD 4:3 frame).
Aspect: "Fill" simply stretches non-HD (4:3) video horizontally correctly eliminating the right/left balck borders, but fails to maintain aspect ratio. Thus there remains large black borders at top and bottom and a distorted video frame (aspect-wise).
An option to stretch VERTICALLY should be provided (even if top/bottom cropping results) in order to maintain aspect ratio when "filling" horizontally.
Actually, an "auto-fill" option should do this stretching both horizontally and vertically as some HD television sets already do.