Well, for one thing, he's bigger, and stronger, and tougher (ever tried to hit a tree? not much happens to the tree, ya know?). His personality is also different, in that he's slower, but more calculating and methodical, less impulsive. And of course he has much more nature related magic at his disposal, and can communicate with plants and animals. And the fact that he's possessed and strengthened by the spirits of twelve long dead Elder Ents.