I'm nearing completion (I'm actually technically done I think, and I don't understand why my Area 51 profile does not yet reflect a fulfilled commitment there) with my commitment for another beta site, and one of my biggest disappointments about it is that the majority of the community seemed to decide very early (even on Day 1 of private beta) that the scope of what kinds of questions were topical was so narrow that although when I committed I had anticipated being able to easily contribute with thoughtful questions, I ultimately was unable to post a question that the majority deemed topical.
I had one question (that I spent >1 hour composing) down-voted 5 or 6 times and having earned 3 or 4 votes to close within 60 minutes of posting before I voluntarily deleted it (which I was surprised to learn earned me a "Peer Pressure" badge; as if caving in to peer pressure was a praise-worthy character trait). And another question I posted was closed within 24 hours. At that point it became overwhelmingly clear to me that I would not be able to post questions that the majority deemed on-topic, and I stopped trying to think of thoughtful questions. I suspect that many other members in this community where 512 people committed went through a very similar thought process, and I suspect that the long-term survival prospects for that community are poor as a result of this rather intolerant mindset on the part of what was apparently a majority within that community.
I even had a few members who (it seemed to me) followed me around the community down-voting every possible post I made. Comments can only be up-voted, so my participation quickly became limited to comments after I contributed my bare minimum necessary to fulfill my commitment. If there had been an uncommit option available to me, I would most definitely have used it because it felt like I was trying to get into a private club that clearly wanted no part of my contributions. This was a rather bizarre feeling given that I had started all starry-eyed and idealistic about helping to found the new community and my idealism quickly turned to cynicism. This also happened to another SE user awhile ago with another beta community, and it resulted in him wanting to uncommit from his beta as well. Looking at the number of up-votes on his question, it seems that there are a goodly number of people who end up feeling this way.
There is apparently no "uncommit" option currently available in the SE software, but I don't think that any new beta community is likely to want to instill the feeling in any of its members that they want to uncommit, for it's not too hard to do the bare minimum required to fulfill one's commitment and then stop contributing (which is, in essence, uncommitting). If many people do that in any new beta then the beta is not likely to survive for long.
NE.SE had 512 committers. If all of them had posted 10 questions, they'd have 5,120 questions in their community. Yet on Day 14 of their beta, they actually have <200 questions. Although I was not in on the private beta for Reverse Engineering, I watched their public beta question-per-day ratio fall steadily down to ~1 question per day on Day 63 of their beta which I feel certain is bound to doom that community. I think these are all very telling details and cautionary developments to nota bene for every new beta which is why I'm posting so much detail in this question about what should be off-topic here.
I think this is a general problem that all new betas are susceptible to. The potential exists for little cliques to form where minority opinions are quickly drummed out of the community by gang-like majorities who are rather intolerant of minority opinions. When that happens, many community members quickly become intimidated and reluctant to contribute, and the long-term effect is that the number of thoughtful questions getting posted to the site begins to fall off rapidly. This naturally leads to an early death for the beta community. So I think it's something that every new beta needs to be especially careful about.