Above is a tree which uses spoke meshes for leaves (used imposter branches for leaves [see wire-frame render in comments]). Below is a tree which uses billboarded quads for leaves.
I'm thinking that mesh type leaves are the way forward. I'll keep support for billboards, they could be very useful for vines, but the next version will not rely on them exclusively. With mesh type leaves the user can choose from a palette of leaf mesh types. Also export to static formats would be possible.
What do you think?