Most ToDo-List services online offer basically hierarchical lists(trees) with some bells and whistles. Mind-mapping software just seems to be a pretty drawn tree.
Is there a conceptual difference? Or not?
From a topological point of view a mindmap is indeed a tree. One difference with a classical tree is that in a mindmap you don't have the implicit prioritization; there's no item at the top of the list, and no item at the bottom.