eeper,
Like someone else mentioned we are allowed to use anticipated seperation. So if the guy that was landing in front of you misses the turn off (and on occasion it happens) you get a go around if I can't get him off before you cross the thresshold.
If the guy that was put in position and then cleared for takeoff doesn't get airborne before you cross the thresshold, LEGALLY I must give you a go around.
But if I'm working the EAA Air Venture air show at OSH (Oshkosh) we work by an entire different set of rules (see the NOTAM) because of the HUGE volume of traffic that we deal with during the show.
Mike