I'm initially filing this bug under gnome-terminal because I'm not sure whether the problem will need to be fixed in it or in gnome-shell. Please feel free to reassign as needed. :-)

Ever since Metacity gained compositing abilities, I've used a transparent background in gnome-terminal. In Metacity, gnome-terminal's shadow effect is projected from the edges of the window, leaving the region behind window's client area unaffected. This means that a theoretical 100% transparency in gnome-terminal's background would display the windows, etc. behind it as-is.

But in gnome-shell, the shadow effect is for the entire window: a rectangle casting a solid shadow - regardless of the transparency of the window's client area. This means that the shadow effect darkens all the items behind gnome-terminal, and so the background of the window's client area is much darker than expected.

Metacity's shadow effect is certainly less "accurate", geometrically speaking, but it leaves the area behind the window untouched. It also means that black text on a semi-transparent white background is readable, whereas the same combination in gnome-shell is hard to read because the background is that much darker, reducing contrast with the text.

I'm not sure what to request: a means to "punch a hole" through the shadow effect; simply changing it to be masked out by default in the region immediately underneath the window; or something else...