First, the secular reasons:
- It's socialism. But...
- The cost is too high. But...
- The government shouldn't tell us who to see. But... (this is a great explanation of the whole issue)
That's all well-and-good, but the biggest issue for me is that we're commanded to care for the poor over and over and over again, and the early church did just that, again and again and again.
The response I've heard is that it is the job of the Church and Christians to give freely, not to be taxed into giving. My response to that is if the Church and Christians were doing their job (myself included), there wouldn't be a need for health care reform. For that matter, there wouldn't be a need for health insurance at all. Anyone who needed medical care and couldn't afford it would be subsidized by the Church.
I think Richard Beck of Experimental Theology puts it best:
Where's the moral outrage in the Christian community about the uninsured? Churches by and large botched it during the Civil Rights Movement. Let's get on the right side of history this time around.