I think it is a very good description.
May I offer a couple of comments on an unspoken part of the message (between the lines), and on the UI itself?
The colors for Waiting and Someday are somewhat similar (at least on the iPad). This might suggest that there is some similarity between Waiting and Someday. Further, the text says you keep your important projects high up and Waiting and Someday projects further down. Again, this may sound to an innocent user as if Waiting is some kind of "half-Someday" (or possibly subsequent/sequential). And the icon for Waiting looks like a pause button!
The way I see it Waiting is just as sure and imminent as Next. The difference is only that Waiting is somebody else's Next, not your own. But it can be just as crucial to your success, and could well turn out to be completed well before any of your own tasks. It is definitely not "paused" or "on hold" or anything of that nature.
I suggest that the colors and the left-menu list grouping should reflect this. Next and Waiting are both "committed ASAP", whereas Someday really means Maybe (not committed).
A Waiting For "project", which you mention in the blog post, is something very rare, I guess, unless you really need to monitor somebody's progress every step of the way, and need to keep their project outline on your own lists. Usually, what I do, is I keep only their agreed "deliverables" (milestones) as individual Waiting For actions on my own lists (often within one of my own active projects).