Actually, it is not a big deal. The DC9, MD80, 727series and I suspect the 757 series all have sliding windows, manual or otherwise. Many times in the summer the cockpit could become quite warm and we would open the windows on taxi (in or out). Also, nice cool spring days it was nice to open the windows and taxi in or out at the smaller airports (i.e. Bluegrass at LEX
)and enjoy the scenery. If the air packs are on when closing the window, it actually acts in the same way as an outflow valve. You just close it slowly so that there is no pressurization bump and nobody's ears notice the difference. If left open on takeoff I suspect that it would become increasingly noisy but should present no problem.....just close it slowly when COMFORTABLY airborne; certainly NOT when rotating or at any critical phase of the takeoff/climb.