Since 13w38c leaves look much brighter because they no longer have shading on their sides and "inside" them, just a little shadow beneath them, no matter the brightness level.

I'm not sure what's happening here so I can't explain myself better than this, just look at the pictures: the first one was taken in 1.6.4, the second one in 13w38c, both at maximum Smooth Lighting and Moody brightness level. notice how the logs are in the dark in the first pic and perfectly visible in the second one.