I've noticed a couple of issues with the Hierarchy window when dragging / dropping.
If you have two sets of parented bones within a hierarchy and truncate one of those sets of bones, then drag/drop an object onto a bone in the open hierarchy set, the truncated set of bones will pop open and fully extend, meaning I lose what I was looking at as the whole hierarchy has extended down the window.
If this doesn't make sense, I can post some screenshots.
Also in the hierarchy window, it would be nice if the item you are dragging becomes alpha'd slightly so you can see where you are dropping the item you are dragging (similar to how photoshop handles drag/dropping layers). At the moment drag/drop can feel a bit awkward because I cannot always see for sure where I am dropping the item.