What is the WPF way to set different template for leafs and inner nodes in the TreeView, if I've got one hierarchical datatype in which the leafs and inner nodes are not differentiated by type?
If I understand correctly, you are looking for DataTemplateSelectors. Here is a tutorial on how to use them :
http://tech.pro/tutorial/807/wpf-tutorial-how-to-use-a-datatemplateselector