There are two conference formats I'm familiar with. One is typified by ACL and ICML; the other by NIPS. The ACL format is a roughly 3 day schedule, with three parallel sessions, which gives time for somewhere around 80 presentations, each 20 mins + 5 for questions. ACL has smallish poster sessions that are typically associated with "short papers." My impression is that many people do not attend the poster sessions.
In contrast, NIPS typically accepts many more papers (120ish per year), but the vast majority are presented as posters. There is only one track at the conference, with several invited talks, a few paper presentations (maybe 15 total) that last 20 minutes. They also have "spotlight presentations", which are 2 slide, 2 minute talks designed to draw attention to particularly interesting posters. The poster sessions run from (roughly) 6 until 10 every night and are attended by everyone.
My feeling is that both formats have advantages and disadvantages. I personally like the NIPS format, essentially because I feel that at the end of the conference, I have seen more of what is there (because I spend so much time at posters). This also means that I'm completely exhausted at the end of NIPS. The major disadvantage to the NIPS format seems to be that I'm used to using post-conference time as dinner socialization time, and this seems to happen somewhat less at NIPS (this is confirmed by a friend who is more in the "in crowd" at NIPS than I am). I think that it might be a little intimidating for junior researchers to go up to a "famous" poster, but on the other hand, this forces you to immediately get to know everyone. The "lots of posters" format also means that the decision about the number of papers to accept at NIPS is essentially limited by physical space, rather than length of the conference. Ken Church seems to think we should be accepting more papers, anyway.
Are there other major disadvantages to having a single-track conference, with the majority of papers presented as posters? I don't expect to see a shift in how our conferences are held, but I'd like to understand all the pros and cons.