The second reason was also pretty clear, but until now I was plugging my ears and humming a tune ignoring it. My entire concept of a good editor was based upon experience using editors that were released for consumption, widely used, and officially supported products in their own right. This editor will only ever be used by me, is applicable only to this specific project, requires no support of any kind, and doesn't have a team of dedicated tool developers to make it their focus. Any effort spent making it into a "proper" product is wasted effort, distracting from the ultimate goal of making a game. The editor exists solely to ease the burden of producing levels, and nothing more.
And that was it. My initial concept of "easing the burden" was by default a full-on GUI app like all the others I'd ever used. It was either that, or just writing a raw text file and throwing it to the engine; there is nothing in between.
Or is there?
I paused to consider the only other "editor" I was using at the time: the WPF designer. It had never occured to me until recently, but in all the time that I've used it, I've NEVER clicked, dragged, instantiated, deleted, altered, moved, or otherwise manipulated a single thing in the designer window itself. Everything I ever did was done in the markup window; the designer window existing solely as feedback for my alterations. All that time, and I'd never considered my burden to be anything but sufficiently eased.
GUI layout is plenty different from level layout, but in the end, my levels are going to be very simple arrangements of basic 4D shapes. Anything so complicated as to require delicate manipulation with a mouse-rich interface, will be too complex for the player to comprehend and navigate in the game, to say nothing of too complex to render in realtime on the 360.
So, to drive this home, I plan to simplify my efforts significantly to a markup description of the scene, and a feedback window to view the results from different angles. While perhaps not as ideal as a proper editor, I feel the burden will be sufficently eased to meet my needs, while freeing most of my architectural efforts to be used on the game itself.
In other words, I'm too lazy and incompetent to produce a proper tool.