Notes as I play:
Interactive tutorial is always nice. Feels like moving the mouse to select a squad order is too sensitive. It would also be nice if I could use the scroll for that. Starting a mission begins on the Options tab, I personally think it makes more sense for it to start on Briefing instead. I beat all the campaign missions without a fuss.
I try out the custom level mode. Not sure if intended, but I kinda like the feature to hold left/right click for selection, then being able to press the other click to do the alternate action. Only problem is if you want to do neither, since it seems you can't cancel the selection out right. Though I suppose you can use undo instead for that. Using middle click for adding doors is awkward, and the hotkey tooltip only works when hovering on its pivot point. On that topic, I think it would be helpful to highlight the pivot point at all times.
Deleting anything on top of a floor tile also deletes the floor. This may be useful sometimes, but would be a hassle while editing the internals of a building. It should really be an option that you check on/off. As I say that I realize checking off "Also add floor" stops that phenomenon. I guess that makes sense, though I think it would be nicer if it didn't delete the floor when you delete the object.
The WASD camera movements become impeded if you select any of the button options(that doesn't open a menu) under the tabs in the top right, since it takes focus. Goes away upon changing tab or opening a menu.
The idea of Wings for randomized mission design is pretty cool, as well as the amount of customization there seems to be.