[monkeyspaw]

Mind maps don't imply a hierarchy. This change in how a problem is structured can alter how you think about it. (It's basically a different data structure, which makes it easier or harder to perform certain processing on it.)

[tunesmith]

Many mind map products do imply a hierarchy, though, specifically because they often do not allow linking a child to multiple parents. While they visually appear to be non-hierarchical, spreading out in multiple directions, they usually are a data structure where each node can have multiple children, but only one parent. This can be represented in an outliner, while a graph data structure cannot.