rog747 - Another TCI problem was not having appropriate procedures and facilities/equipment in place to operate more than one aircraft at any one time safely in low visibility. The Milan/Linate runway collision in early 2000's involving the SAS MD87 and biz-jet crossing the runway again highlighted this where the biz-jet took a wrong turn, ATC couldn't see the aircraft, inadequate signs, lighting etc.
Regarding close parallel runways used in segregated mode, another capacity constraint is if an arriving aircraft on one runway needs to go around and one is departing on the other runway at the same time then there is an immediate risk of airborne conflict.