Now, a round-table discussion is a different beast, as are single-presenter talks before a specialized audience; in those cases, the Q&A is often a great opportunity to hear someone expand on her work or for the group to brainstorm together. But the average conference Q&A involves what seems to be an invisible bucket of nouns passed around the audience. Each person sticks her hand in, pulls out a random noun, and builds a question around it.
The wind-up will vary in length and coherence, but the question usually amounts to, "I'm wondering if you can say anything about how your research engages with [RANDOM NOUN]."
You can make your own set and play at home, but in my field the cards might include:
Deleuze & Guattari
The Bishops War
Book 2 of The Faerie Queene
Sometimes the nouns are extremely specialized and sometimes they're extremely broad, but the common feature is that they come from left field and may be of interest to literally no one but the person doing the asking. In the most egregious cases, the questioner will grab a whole fistful of nouns and string them together--not always with any logical connection and not always phrased in the form of a question--followed by, "anyway, I'd love to hear what you think about that."
Four times out of five, I hate sitting through someone else's Q&A session. Nine times out of ten, I hate participating in one as a presenter. Partly this is because I don't process information well aurally, so though I've had some very successful Q&A sessions, they still felt like artificial exercises. Rarely have I actually had any new thoughts in the process of the session; I've just succeeded in sounding smart because the questions gave me the chance to explain the background or larger context of my project or to rehearse material from the longer version. I give potted answers, basically, even if they don't sound that way and weren't prepared in advance.
Increasingly, these days, when someone heaves a bucket-of-nouns question at me, I just say, "to be honest, I haven't thought about my work in connection with X." Or "Well, I haven't read that in a long time, so I shouldn't pretend to know more than I do." Sometimes I'll give a version of, "I'm not answering that question" four or five times in a single Q&A.
I value real questions and real feedback about my work, and I love talking with audience members about it afterwards. But even the best questions are not best asked or answered in a relatively high-pressure situation--and in my experience, the best questions account for maybe 10% of the questions one gets asked.
The rest? Bucket of nouns.