From what I have seen, it appears to go like this: each departing plane gets a "slot" - a 5 minute window in which to pull back from the gate and get into the takeoff queue. If a plane misses its "slot" - even by only a minute or two, it gets dumped to the back of the queue. (And at IAH, gets shuffled to the most remote runway.) The plane then has to wait for all the other planes who made their "slot" to takeoff before being allowed to proceed. I am not sure what other factors would keep pushing the plane further back in the queue (flight path congestion?), and I have no idea how weather-related delays impact the "slots" (except that they seem to screw the slots up).