[ansible]

When I first saw them I thought transparent terminal windows were the coolest thing. Then I tried to use one...

So, as other commenters have pointed out, using blur and/or 80% opacity helps with this. One thing I haven't seen (with this terminal or any other) is to actually outline each character in black (assuming white on black color scheme). That would give some much needed contrast, while still allowing areas with no text to be visible without much or any blurring and perhaps not needing the opacity set so high.

[edlinfan]

Eterm does this. It does help, but it is kind of ugly.

[ansible]

I just tried this and I wasn't wowed by the effect. Back to a solid black background for me...