the canned leaves from tree generation programs is the main giveaway for cg. when i need foliage, i'll use photographic clip maps, or trees from a hi-poly/hi-res tree model set i have. then, i'll add foliage created with maya paint effects (these are awesome!!). all techniques i mentioned here have organic style leaves.
another idea would be to incorporate transparency maps to break up the hard edges; by setting transparency to 100% and applying as a fractal procedural shader, you'll cut away at the geometry edges (and bits of the inside, too -- which is ok) and alleviate the "canned" look.
the last idea is to apply a leaf image as a UV map to the leaf geometry, and a transparency map to "cut away" the outside areas around the leaf. then, every leaf would look organic and photorealistic.