I hear you on the car rentals. I've had long waits at rental counters as the staff are limited and there's no one else around to pop in to help. I find the hotels there's usually a third person sort of floating around the front desk to ease the congestion. Although it's always a pain if you just see them wander away!

I will add another reason for delays: chatty people. This happened to me just last week at a car rental counter. As the whole process was going through its motions the counter guy and I were making fun chitchat but then I noticed people waiting behind me in line so I felt the need to wrap up the pleasantries. It probably helped make my transaction speed along but if I was the one waiting I'd be thinking 'can you guys just shut up!'.