I simply haven't been consistent when entering keywords. Some of them are nested in a hierarchy, some of them are flat, just top-level keywords. So I'll have top level "Hummingbird" as well as a nicely organized "Animals>Birds>hummingbirds" and I might also have top level "Sparrow" and no such sub-keyword under "Animals>Birds".
Totally my fault--bad discipline. What I'd love is a nicer way to move them around, for example move Sparrow to Animals/Birds/Sparrow. Obviously I can drag it there, but if you have, a lot of top level keywords, it's pretty tedious. Once I drag something off the bottom of the visible scroll list, the list scrolls amazingly fast, so before I can react, I'm at the bottom of the list. Really great if I need to move items to either the top or the bottom, not so good for moving it to a group in the middle.
It's actually much easier to re-keyword all "Hummingbird" photos as "Animals>Birds>hummingbird", then delete plain "Hummingbird" as a keyword. I may end up using that method.
But to answer your question, the tool I would love to see would be a two-pane view, somewhat like Windows Explorer, where I could set one pane's focus on the group I want to move things into, then scroll the other pane to items that need to be moved, and drag them to the keyword group in the first pane, without having to scroll while dragging. It'd still be some work, but I can imagine actually tackling it.
And yes, this is now way off the original topic