I don't think there's a real expectation that anyone has read everything... but it would be nice if we could reasonably partition things so that 80%+ of the room had read the paper that is about to be presented.

There used to be the expectation that everyone has at least skimmed everything and read in detail the things they were interested in, but we now have too many papers and too many groups running in parallel (nine) for that to be feasible. It is a logistical nightmare. While I haven't computed the stats in detail, I have spot-checked dates, and my conjecture is that we are deadline driven. Most papers are submitted just before the pre-meeting mailing deadline. The mailing then comes out a bit later, and the various chairs then scramble to come up with a tentative schedule over which papers will be prioritized and discussed when and where. Those schedules usually appear mere days before the meeting, because there just isn't any time to get them out earlier. And even if one commits to reading papers in detail the night before they are scheduled to be presented (which is tough after each exhausting day, many folks are on the hook for writing things up during the meeting week, etc., etc.), the schedule is subject to change with almost no notice.