As disastrous and unjustified as the Iraq War may have been, there wasn't anything unconstitutional about it. There was no formal declaration of war, but we haven't gone that route in a while, and both houses of Congress passed an Authorization for the Use of Military Force Against Iraq. Whatever else it was, it didn't represent an attempt by Bush to undermine our whole system of government.
The real Constitution-shredding lay in Bush/Cheney/Addington/Yoo's claims that when acting in the interest of national defense (as determined by him, of course) the president doesn't have to obey the law. That was the essence of the FISA controversy, which was staggeringly soft-pedaled by the media. It was the most dangerous, though hardly the bloodiest, legacy of the Bush administration.
So Obama's decision to basically spit on a court order and claim executive powers he doesn't have is really bad news. The "Unitary Executive" theory is completely un-American and we shouldn't be letting Obama trot it out when he finds it convenient. In the best case scenario, he's only doing it to protect Bush administration officials from prosecution (enforcing the law being too "divisive" for his liking). But even that would be a piss-poor justification, and the alternatives are all worse.