I am something of a meeting nazi. Late in life, I discovered what it means to run a meeting. It means that you are the agent of the desires of the meeting participants in toto. They do not have the power to change the topic; you do. They do not have the power to end the meeting; you do. Your position comes with a responsibility to those on whose behalf you act. A good chairperson runs a meeting according to the Golden Rule: Preside as you would wish to be presided over.
When I preside at conference sessions, I rule with an iron hand. I announce ahead of time how much time each paper will be allowed, give five minute and one minute warnings, and do not hesitate to cut the speaker off politely when they are out of time. I moderate discussion by mentioning at the outset how many minutes are available for the question and answer period, and announcing "one more question" when there are only one or two minutes left in that timeframe.
When I run meetings, I move through the agenda steadily, not allowing early items to eat up all the time available. If discussion is not over when it's time to move on, I invite the participants to continue through e-mail or on our forum. If issues are brought up out of turn, I ask that discussion be delayed until we get to that item. If issues are brought up that are not on the agenda, I ask until they be held until we reach the time for consideration of other business at the end of the meeting.
I do this because I have been to a lot of meetings and conference sessions where the presider acted like he had no power whatsoever. Such moderators believe they are helpless in the face of rude, egotistical, or just well-meaning but clueless people who don't stick to their allotted time or pay attention to the agenda. Their apologetic impotence tends to infuriate those being presided over, whose time is being eaten up, wasted, imprudently rearranged, or simply disregarded by a few participants.
The greatest courtesy one can pay to the people attending your meeting is to respect their time. That's why I act like a polite dictator when I'm in charge of the meeting. A successful meeting is not one where everybody gets their say on everything on the agenda; a successful conference session is not one where the presenters hold court for however long they choose. Nor is such a conference session even possible, considering that each one will end on time no matter how poorly the time has been managed within it -- usually meaning that the poor sap whose paper was scheduled last ends up with ten minutes and no discussion for her twenty minute paper. In a meeting, more's the pity, sometimes there's no end in sight. So the moderator in a conference session must act decisively to preserve the time of the last presenter, and the chair of a meeting must act decisively to keep the meeting within its announced parameters, lest we all be trapped indefinitely.
I don't know how other people experience my benevolent reign of terror in meetings, but I believe the majority of them are grateful. I adopt that persona not because it comes naturally to me, but because the best meetings I've attended have been run that way, and the worst have not. I'm happiest in a meeting that's somewhat overstructured rather than not, happiest in a conference session where I can relax and listen to the papers rather than worrying that Professor X is going to end up with a truncated presentation, watching attendees streaming out in droves as his time expires. Someone taking charge on my behalf -- that's what I want when I attend a meeting. So that's what I try to do when I'm at the head of the table.