Why do pilots disable the air-conditioning 'packs' during takeoff?
The "packs" (or A/C on the aircraft) are powered by engine bleed air. When bleed air is extracted from the engines, less thrust can be produced. This may be a problem if the airplane is taking off at a high elevation airport or the departure path requires a certain climb gradient to clear obstacles. This action does create some discomfort in the cabin so it's usually not preferred unless required for safety.

Some newer airplanes are manufactured with an "APU to pack" configuration - i.e. the pilots leave the APU running during takeoff, which provides bleed air to power the packs. The workload on the engines are now reduced.