In Glasgow we don't have any 'mega' churches. If I'm right, we have a couple of large churches (congregations numbering from 400-700), but thankfully none of Joel Osteen proportions. When I hear of churches in the US with congregants numbering in the 1000s, I scarce can take it in. Even the neo-Reformed (e.g. Driscoll - whose preaching abilities I admire greatly) seem to have bought in to the 'mega church' model.
In my opinion, it is near impossible for a large church to function as it should from a NT standpoint. First and most obviously, how do you administer communion in a setting with thousands present? How do you guard the Lord's Table from unbelievers? To abuse the Supper is to invite judgement upon your assembly, yet it must be nigh impossible for ministers/elders to be on their guard when giving out the Supper weekly to faces they've never seen before.
Secondly, how does a mega church 'do' discipline? I suspect that most don't. Nevertheless, it is easy for those needing church discipline to disappear into the throng. Hundreds could be involved in sin that would make Ted Haggard blush, yet they'd remain unaccountable, while swaying with eyes closed singing love songs to Jesus. That's not to mention those who need discipline for non-attendance. How do you even tell who is and who isn't there?
Third, what about mutual accountability and loneliness? Church is a place for the downtrodden, the low in society, the depressed, the socially awkward, etc. The kingdom of God is for such as these. How are people to form godly relationships, to be 'forced' out of their little shells into loving mutual accountability and to find loving support in such a massive crowd environment.
Fourth, what about the problem of the biblicists? Big churches attract wackos who've been punted or moved from church to church. Most big churches employ the use of 'house groups' to solve the relationship problem I mentioned in my third point. During these house groups, any Charles Manson with a Life Application Study Bible can voice his heretical/biblicist views.
Fifth, big churches create celebrity pastors. If I see another pastor on TV with a radio mike in his ear and fake tan rubbed around his surgically enhanced jowl's I'm going to, in true Elisha style, pray for some angry bears. I'll also pray for locusts and point them to the glossy church magazines. Large crowds feed the celebrity aspirations of many pastors and church becomes all about their 'performance' rather than a place where the saints are fed with the body and blood of the risen Christ.
In my opinion, churches start to lose their intimacy and ability to function effectively when the congregation reaches between 200-250. Imagine a church that planted another church every time the roll reached 200! I'd much rather have a city with 30 churches of 200 people than one with a mega church of 6000.