Vegreville has put up a short (but good) piece on the difference between academic seminar/presentations and "bad" meetings.
Here's a follow up to one of his points - occasionally you have an individual in the audience that hijacks the presentation with one question after another (or gets in an argument with the presenter). Since I recently read a piece in the Wall Street Journal on Ben Franklin, here's some advice from him I came across a while back. I repeat it here more to remind myself than anything else, since I have a thick head that processes and retains new information very, very slowly.