We've talked a lot about how theological affiliation (particularly liberal theology) may affect the overall decline of mainline denominations. But beyond the stated theology of the church, I think that there are two other major factors that have induced the decline of mainline denominations:
2. Failure of churches to change their languages of communication. As much as I suspect that the emerging church movement is a marketing conspiracy of the haircare industry and Starbucks, this movement at least fully realizes that the Gospel is not bound to any one culture. There is no one way of "doing church", but so many local churches are committed to staying in 1950s Americana. This problem encompasses the whole contemporary worship vs. traditional worship struggle, but also means completely thinking outside of the box in order to stay focused on the mission of the church. Wherever God's children are, we go.
Personal example: when I was growing up, my family moved a lot. We usually went to UMC and PCUSA churches. And I've met a lot of people who typically moved between these two denominations. I recently asked my parents why we did this. They said, "Well, they're just alike; the two denominations are interchangeable." I knew what they meant -- both embraced a particular style, form, and social class. But the two shouldn't be alike! Calvinist orthodoxy and Methodist orthodoxy are very different. The flavor of the sermons, teaching, and ecclesiology of the two denominations should be different. But they weren't different in the way of "doing church" -- church as a cultural institution.
3. Which leads me neatly into the third factor leading into mainline decline: the end of the Constantinian era. There is no longer a social obligation to go to church. As much as this has led to mainline decline and what people bemoan the "post-Christian age", I thank God that this social obligation has ended. It was a cultural force which inoculated countless people from a transformative faith.
Again, personal example: For my parents, born in the Deep South in 1940 and 1942 respectively, the idea of not going to church was inconceivable. Growing up, I thought that being a Christian meant showing up on Sundays and dressing neatly. When I first went to church at the age of 25 after nearly a decade of absence, I was absolutely shocked to see men not wearing suits. But although my parents are deeply moral people with a stern and private faith, what I learned was that being a Christian meant "going to church." Any emotional religious expression should be kept private and solitary. But going to church was optional for me, and removed from Constantinian constraints, I stopped going. I don't blame my parents -- by no means. Constantinian social dynamics had turned them into serious Christians and they had no reason to believe that they wouldn't work for me and my brother. The end of Constantinian America caught them completely off-guard, and is still catching many churches off-guard.
The Constantinian era was poison to Christianity, and if the consequence of losing it is mainline decline, so be it. Perhaps, as Bruce Alderman eloquently writes, the loss of numbers isn't all that important. Perhaps. It depends on why we are growing or shrinking. I see far too many elderly United Methodists who know that the Titanic is sinking but are content with leaving the deckchairs in their current arrangement because it won't sink until after they die. It wouldn't surprise me to learn that this is the situation in many other mainline denominations.