Appeal to authority. If someone tells you about their job or other qualifications before asking a question then the question is almost certain to be useless. If a question is good then it can be asked by someone with no special qualifications who has 20 seconds to ask.
An introduction that shows that it’s not a question. Anything starting with a statement like “I just want to say” isn’t a question and has no place in a conference lecture hall. After a lecture the speaker will usually hang around and talk to delegates, anyone can make comments then or send email later. In this case it’s not just that statements are inappropriate for “question time” it’s that people who think that they are so important that their statement is more important than genuine questions probably aren’t going to have anything useful to say.
A second question. Anyone who has more than one question wants a conversation – they can do that privately after question time. Again it’s people with an over-inflated opinion of their own importance who do this.
A statement of “fact” that they want the speaker to address. Questions should mostly concern facts referenced by the speaker. “Facts” that are cited by the audience are often the sort of thing that can be easily disproved by Snopes or Wikipedia – but not in the amount of time available during a lecture. While it is possible to ask useful questions regarding facts that weren’t presented in the lecture my observation is that most such questions are worthless and the “facts” are false.
I think that for a serious lecture the MC should cut off such questions when they start. Once enough has been said to make it obvious that the question falls into one of the above categories I believe that the correct thing to do is to say “that’s probably a good thing to discuss after the lecture”, and then move on to the next question.
Please note that the above list isn’t comprehensive. Let me know if you have suggestions for any I missed.