One of the stranger things I've noted at IAD is that, if it's your turn and you've got a situation that's going to take some time to resolve (involving a call back from someone elsewhere), the agent doesn't move on to the next person... they just sit there and wait, with you, while nothing happens. I grew a bit uncomfortable about this and after at least 5 minutes of nothing happening, I suggested that perhaps she help some of the other folk in line (my flight wasn't leaving for a couple hours). She said no, that's not how things work. I probably held up the line for 15 minutes, for no good reason that I could tell.

My experience at SFO and ORD have been that an agent's more than happy to essentially put you on hold while they take care of a few other passengers while waiting for something. I have no problem with that, especially if I'm not in danger of a tight connection being lost.