Lance Fortnow asks if theoretical computer scientists are too nice.
...[Theoretical computer scientists] (outside of crypto) rarely ask negative questions in a seminar. Typically the only negative feedback we get in our field is from anonymous referees and reviewers. If we were forced to defend our research in an interactive setting, we would establish a better understanding of the importance of the models and results of our own research.
This endemic “niceness” goes well beyond theoretical computer science. It's extremely rare for seminar or colloquium speakers in any area of computer science to receive negative feedback, even in situations like thesis defenses and interview talks where direct challenges are perfectly appropriate, if not desirable. (Of course, there are exceptions. I was the victim of a “designated jerk” at one of my job interviews.)
Update: Suresh disagrees, describing his experience at AT&T as evidence against the “message from God” model. He raises a good point about conference talks: A big part of why they don't get much negative feedback is that most of the audience doesn't understand the technical details—although this is much less true at more specialized conferences like SOCG—and there isn't time for discussion anyway. And yes, discussions are usually much more spirited in more focused seminars. But then what about interview talks? Maybe it's just my department that's too nice!