I think it depends on your goals.
Have you already decided you want the job? "Based on this interview and the information I have given you, what do you see as my strengths compared to other candidates?" Same question for weaknesses but think about not highlighting them unless you are pretty sure you can defeat them by addressing them. One way to ask about them would be to ask something more leading, like "what are you unsure about, where you would like to hear more from me about the topic?"
Are you really just thinking about the job, where you already have adequate employment? Then ask about what matters to you in a job.
Listening to a talk for fun or learning? I think contextualized questions about application of ideas are good to bring the abstract down to the concrete and sometime reveal more. So, you might start a question with, "I am a ___ and in that work I see a lot of ___ . How do you think your idea of ___ applies to a ___ where ___ is usually the case?" I think the key there is not to focus on the introduction. Most people like to talk about themselves, so it is easy to fall into that trap and forget about asking a question.
I had a job interview earlier today for a job that I've already decided that I want. Having recently read this post, I tried the "what do you see as my strengths" question in the field. I don't think it went well. It seemed like a very thinly veiled attempt at making them say positive things about me and I suspect the interviewing panel was smart enough to realize this. The result, to my perception, was that I came off as the kind of person who would attempt to deploy cheap petty psychological tricks. Rather than calling me on this, the interview...
I think it's past time for another Stupid Questions thread, so here we go.
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Please respect people trying to fix any ignorance they might have, rather than mocking that ignorance.