Here at a secret military airfield in West London, the majority are IFR departures and as such are subject to slot times/CTOTs etc. Most of the delayed departures are down to the fact that the surrounding airspace is just so busy! If an aircraft is sat waiting for his release, we will endeavour to explain the cause of the delay. I am pleased to say that most pilots are quite understanding and accept that we are doing our best for them. After all, we find it just as frustrating - especially when it's the last movement of the day....
That said, I have little sympathy for the guys that really cut it fine and then grumble at us when they have missed their slot and are subject to huge delays because their pax have wombled in at the last possible minute.
We DO have a pecking order, ie Mil/Civil/VIP/ODV etc etc, this being a military airfield, but that is the nature of operations here.