I would like the ability to "freeze" specific parts, features, or faces, of a model to prevent modification of that part, feature, or face. In my experience with history based modelers, this is not an issue. If you want to move a feature, you modify the sketch, and only that feature will change. When direct modeling its hard to keep tight dimensions if you move one face, and it causes an angle to be changed, or a hole size to changed. I would like to be able to choose and lock the features that must not change.