You have a good sense of general volume of the leaves.
pay attention to your ref if you have one, try to replicate the hardest things. Notably, virtually all trees (and many shrubs) have non-uniform leaf coverage (which results in partial or total holes)
, so break up the uniformity of your shading
And, consider the volumes in relation to each other.. like you have put together three different lumps of clay.. they change angle gradually, so the light level should match between eg. the back edge of the frontmost blob, and the front edge of the backmost blob.
Quick edit:

EDIT: Also, I agree with Jad's comment below. you need to decide the normal leaf shape and fix the outline shape to match it.Keeping in mind that the leaf shape is likely to be skewed near the edges of the outline because it's facing at an angle to the viewer.
« Last Edit: June 18, 2010, 08:37:30 am by Ai »

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If you insist on being pessimistic about your own abilities, consider also being pessimistic about the accuracy of that pessimistic judgement.