Kevin Drum makes a very good argument regarding cap-and-trade versus carbon tax; the proponents of carbon tax can pretend their option has no loop-holes or unnecessary complexities precisely because they haven't gotten anywhere with it. Specifics only matter once the general concept has been accepted, or at least gotten somewhere near being accepted.
I'm not sufficiently up on the specifics to be sure Drum is right, but it's a very persuasive argument, and it's a reminder that a good plan poorly implemented is far better than a perfect plan that can never be implemented at all. This is something Liberal Democrats in this country and progressives in general should remember, to ensure that we don't keep fighting imperfect plans because we imagine a vastly superior alternative that will never occur anyway, at least without dilution to the point that it isn't much better in any case.
In short: anyone can promise they'll heal the sick upon election if they know they'll never, ever win.