I've always been impressed how the SIGCOMM conference accepts a very small number of papers, but many hundreds of people attend the conference. (This is true for some other conferences as well; SIGCOMM is just he one I happen to know best.) The conference is somehow an "attend-if-possible" venue for the networking community. Most other large conferences I know work instead by accepting a lot of papers. ISIT is a week-long event with 9 parallel sessions (in 2014). Location may certainly be helpful; ISIT was in Honolulu in 2014 and Hong Kong in 2015; SIGCOMM will be in London this year. But it also isn't decisive; the Allerton conference has been steadily growing, so it's a few hundred people, even though Urbana-Champaign, where the conference is held, really can't be the biggest draw. (No offense to those who are there -- I always enjoy visiting!) In fact, really, at this point, Allerton has been so successful it has seemingly outgrown the Allerton conference center!
If you have ideas for what makes a conference a "must-attend" venue, I'd be interested in hearing them. The question has arisen because of a Windows on Theory post about changing the format for STOC 2017; for those of you on the theory side who haven't seen it, I'd recommend looking at the post and registering an opinion by comment or by e-mail to the appropriate parties, after you've considered the issue. The underlying question is what role should the STOC/FOCS conferences play in the world these days, and how might we change things to get them to play that role. I think another way of framing this is that I don't think STOC/FOCS aren't really "attend-if-possible" venues for much of the theory community -- particularly for certain subareas -- and the question is whether this can change. Again, I'd be happy for insights from other communities as well.
There are a large number of comments already. I'd say in terms of my personal opinion on what to do I'm most aligned with Alex Lopez-Ortiz and Eric Vigoda. Alex's point is straightforward -- accept more papers. I've been in the accept more papers camp for a while (a very old post on the topic suggests one way it could be done), but for some reason there seems to be huge numbers of people in the theory community against the idea, based on some sort of quality argument. For me, the tradeoffs are pretty simple; I generally prefer not to travel, so I need a good reason to go to a conference, and one very good reason is that I have a paper in the conference that I or a colleague is presenting. (I recognize this is not true for everyone, but being selective in travel is an issue that arises for people with families, or other regular obligations.) Clearly conferences such as SIGCOMM show that there are other paths possible, but I'm not clear on how to reproduce that kind of buy-in from the theory community. Eric's suggestion was that lots of theory conferences happen during the summer, so instead of trying other ways to create a larger "STOC festival", why not co-locate a number of conferences (that would have parallel sessions) that would get more of the theoretical computer science community together. That makes sense to me, and seems like it could be particularly beneficial for some of the smaller conferences.
But my personal opinion, while undoubtedly fascinating to some (most notably myself), is really not the point. The point is for the community to examine the current situation, think if there are ways to make it better, and then act on what they come up with. The bet way for this to happen is if people actively participate in discussions. So please, go opine! There's no reason the post shouldn't have 100+ comments, so comment away.