JCCs around the country have been doing the same things as megachurches for a really long time. Here, under one roof, let's put all the activities of life so that people can come and stay and be with their village. Our youngest goes to preschool at the JCC, I work out there, the kids swim there and take art classes, I go to Mom's Night Out, and the list is endless. After some remodeling is done, there will be a Starbuck's and a manicurist. If we weren't moving, I might quit my job and just hang out at the JCC all day. That's the point.
I can do her one better--my actual synagogue is currently housed in our local JCC (long story involving a breakaway from our old congregation). So we really do encompass the whole nine yards inside one building, just like a "real" mega-church (though we're much, much smaller).
That all being said, I think that much of the megachurch critique centers around a fear of southern church radicalism, not really anything about breaking down "communities" (though that may serve as a good fig leaf). Though most northern liberals, I think, harbor a latent mistrust of southern religiosity in general (buttressed by the anti-gay and anti-liberal mentalities associated with them), I think most also feel that pluralism is a check--with many churches and pastors to choose from, people will be exposed to different messages and perspectives that will create a more heterogeneous community. Combining everything into one megachurch also means eliminating many of those voices and centering the entire religious message around one uber-pastor (and perhaps a few of his associate preachers). All the articles I've read about megachurches include at least something about how the pastor is in many ways like a politician with various constituencies he has to manage. One of those constituencies (and, I'd wager, normally a quite powerful one) is the radical religious right. And my guess is that this group would thus wield a disproportionate amount of power in determining the church's message on issues of political importance--gays, poverty, war, abortion, et al.