With a bit of tweaking, I think that's a great idea! Doubt it'll come to pass though!
It does sound a good idea until you scratch below the surface.
For one thing I don't think it should be mandatory for all, apart from the stated exceptions - which is the main objection to it. If a person finds employment in which they are happy, or are embarking on a training of their own choosing, are they seriously going to want to give it up for a year to do something they might not want to? Would employers be prepared to keep a job open for that long, which they were obliged to day in the days of the old National Service, but things have changed, and, as I see it, one of the tweaks would have to be that, if an employee wished to take advantage of the scheme, his or her job would be protected. The one advantage of course might be that a temporary job would become available for somebody else, depending on the type of work involved.
What guarantee would there be anyway of unemployed participants in the scheme finding a job when that year is up, given today's employment climate?
Also I can't see your average moronic yob finding the scheme attractive. All that would happen is that they would be coerced unwillingly into something, and 9 times out of 10 would merely continue causing trouble.
Apart from the military, where would all these people find the necessary work, whether voluntary or otherwise, along with decent accommodation - which is a valid reason for not forcing people to leave home or other permanent accommodation they occupy (another major objection)?
Perhaps a similar scheme which operated on a non-mandatory part-time basis would be more acceptable. It works for the Territorial Army where Terries are usually in a full-time civilian job or training, so why couldn't it be applied to other areas, especially as a lot of voluntary charitable work is already part-time.