(limited # of gates so waiting for flight scheduled in front of your flight to leave/arrive, 4) Airplane availability, 5) Mechanical (both your plane and a plane sitting in front of you at a gate), 6) Technical (computer issues by the carrier and even sometimes the airport), (6) Crew timing out including steward/esses, 7) loading cargo (many passenger flights carry freight too), 8) angry passenger delaying takeoff.

I fly a lot and have experienced each of them multiple times. #'s 1 and 2 are by far the most common and cause the most late flights and cancellations. #3 delays a lot but less than #1 and #2 and a lot more than the following #'s. 85% of my flights have arrived within a reasonble time (30 minutes). I usually fly Frontier because of its much lower cost but when I must be somewhere at a set date and time I'll fly Southwest, United, Delta, and American in that order.

Book flights as early in the morning as possible to avoid propagation of late flights because of earlier late flights.

Direct flight does not equal non-stop. It means a layover sometimes short - even the same plane - sometimes long. The second flight is usually held for the first flight to arrive if the first flight is running late. Greater chance of a problem but not much.

Happy flying.