I think it is more of a battle of attrition than any absolute temperature that would automatically cancel flights. Maybe there is some absolute limit but that may be -20F or colder.
But as you mention, people will need to take more breaks to warm up, gradually as temperatures get colder the de-icing and ground crews will end up bottlenecking the flights so some will end up being canceled premptively in order to manage the overall system.
One article popped up with a search:
http://www.businessinsider.com/why-planes-cant-take-off-in-cold-weather-2014-1
If you consider Minneapolis as an example, it gets colder there than Chicago and all the airside equipment still works into the extreme temperatures.
Moisture however is another story, ice and snow will have a larger effect than the temperature alone, especially at Midway since the runways are a little shorter than other airports. Chicago Department of Aviation has to be able to keep the runways clear as there isn't as much extra allowance for braking distance, and visibility is needed for the approach for landing.
Home airport MDW, frequent visitor to MCO to see the mouse.
