It lookks like there is some texture filtering applied, and this migh be the reason why leaves seem to have lost some transparency, creating this "blocky" look. Check the render options, and in front of Object antialiasing, click on Edit, and in the new window, check on the right side if there is texture filtering applied. If so, setit back to zero. and rerender.
If you used a preset, any present above "final" (but not final) now uses tetxure filtering.