This is probably old news, but I've been working a bit with plant models lately and came up with a method to really improve the photorealism of leaves, fronds, etc.
I've always disliked the hard-edged shadows one gets from leaves, etc., even with soft shadows enabled. Another problem is the deep shadows in clumps of leaves - they tend to be too dark. In real life, leaves aren't completely opaque. If you hold a leaf up to the sun and place your finger behind it, you can see the shadow of your finger through the leaf, right? So, both the hard edge of the shadows and the unrealistically dark shadow deep in dense clumps of leaves might be addressed by making the leaf pass some light through. You can kind of fake the proper effect by tweaking the translucency of the leaves a bit, but it's not ideal.
Most plant models come with opacity masks for the leaves. They're usually hard white on black. I took a set of leaf textures from a model and darkened the entire image by ~50%, so pure white became a medium gray (RGB = 128,128,128), and the limited number of existing gray fade pixels on the opacity image retained their relative transparency compared to the previously completely opaque regions.
The result was what I was looking for. Probably not everyone will like the look, but I do. Check out the attached image. The tree on the left is the unretouched model, the one on the right has the opacity tweak. (Both are the same model.) And look at the shadows on the ground. Personally, I think it's a big improvement. It does make the leaves a little brighter, but you can dial down the Kd value below 0.8 (the default) to compensate. (I liked the lighter green on the one I modified, though.)
