Please allow to turn artboards
Allow to turn artboards just like vectorial shapes vertically or horizontally, no more having to enter the opposite measures on each size tab.
Egor Chistyakov commented
We have Flip command in Transform panel fly-out menu disabled when Artboard tool is active, and all Transform panel is disabled for whatever reason when you deal with artboards.
I'd treat artboards as special live shapes: you can set common width and height in separate set of fields (will affect to all of selected artboards), or set common coordinates, but you can also should be able to use classic X/Y/Width/Height in Transform controls, to scale, reposition and do whatever you want with artboards.