Now that this is turned into a community wiki post, I'll throw an answer out there: No, a non-answer!
And my non-answer is this: There are too many conferences, and the vast majority of them too specialized for most of us, to answer this question in a way that would be useful to most mathematicians. (Yeah, I am sticking my neck out here, speaking for “most mathematicians” when it's not altogether clear that I am competent to speak for myself.)
I tend to prefer narrowly focused conferences myself, in a field that I actually know something about. That way, I can at least feel appropriately ashamed when I can't follow a talk on a subject I had thought I understood. Which provides an incentive to do something about it. So if I may add to the original question, then? Why do people go to big general conferences? What do they get out of it? Okay, this may sound too negative. Breadth is good, and I wish I had more of it, but mathematics is a vast field and only a few of us can attain deep understanding in more than a couple of subfields, at best.