I generally agree with Glenn that when the executive takes that kind of power it is likely to expand it. Obama has done so with drone strikes. Bush did so with numerous policies. And history is riddled with similar instances.
I made kind of the opposite argument about the power congress has in relation to the individual mandate. I said that just because it can force people to buy health insurance doesn't automatically mean that it can then claim the power to force people to eat broccoli. Further, I think the supreme court should allow congress to use that power because it's not likely that the adage Glenn was talking about would follow in relation to congress' power.
Am I contradicting myself when I defend the expansion of a power by congress but condemn an expansion of a power by the executive? I don't think I am. There is always the possibility that a given institution will seek to expand it's power. The founders thought it was so likely that they built that assumption into our system of gov't. And aside from electoral checks on power, the separation and sharing of power between our branches of gov't serves as a check on power. Basically, ambition checks ambition.
The reason I am less inclined to embrace the kind of executive power Bush and Obama have sought and largely gained is because it goes mostly unchecked by congress and the courts. The executive is afforded more power of foreign policy and national security than congress, and for some good reasons. Congress is still supposed to check that power. But given that most of it is inherently vested in the executive history seems to show that it's difficult for congress to check that power and refuse expansions. In current cases, the very nature of the power is secretive, which they argue is needed because it pertains to national security. So that makes it even harder for congress to check executive ambition.
I could get into the specific policies in question and compare the negative consequences of them. Briefly, being forced to buy health insurance is much less an intrusion on freedom than having your phones tapped and private messages read without a warrant. But even though I think that would be the case on average, I think the different dynamic regarding ambition being able to check ambition is enough to merit more caution in foreign policy/national security issues.
Update: Here's a post from Conor Friedersdorf about Abraham Lincoln's views on the executive power to wage war:
Allow the President to invade a neighboring nation whenever he shall deem it necessary to repel an invasion, and you allow him to do so whenever he may choose to say he deems it necessary for such purpose, and you allow him to make war at pleasure. Study to see if you can fix any limit to his power in this respect, after having given him so much as you propose. If to-day he should choose to say he thinks it necessary to invade Canada to prevent the British from invading us, how could you stop him? You may say to him,--I see no probability of the British invading us"; but he will say to you, "Be silent: I see it, if you don't."
The provision of the Constitution giving the war making power to Congress was dictated, as I understand it, by the following reasons: kings had always been involving and impoverishing their people in wars, pretending generally, if not always, that the good of the people was the object. This our convention understood to be the most oppressive of all kingly oppressions, and they resolved to so frame the Constitution that no one man should hold the power of bringing this oppression upon us. But your view destroys the whole matter, and places our President where kings have always stood.