"I won't just ask for your vote as a candidate; I will ask for your service and your active citizenship when I am president of the United States. This will not be a call issued in one speech or program; this will be a cause of my presidency.”— Barack Obama, December 5, 2007

Broken down a bit by Jim Lindgren...

So – if Obama means what he says – his civilian national security corps would cost at least another $100 billion a year, and perhaps as much as $500 billion a year. With total federal income taxes of $935 billion in 2005, Obama's proposal would mean using up to half of all federal income tax revenues just to fund his promise “to have a civilian national security force that's just as powerful, just as strong, just as well-funded” as the military.

Three quick reasons to fear this even if you really like the guy proposing it:
1. Cost.
2. Invasiveness- what part of the country wouldn't be covered by a corps?
3. Imagine this army of "armies" under the control of a president you don't like so much.