Here's why ID is inherently supernatural, and thus, not science.
1. Everyone knows that this "intelligent designer" is supposed to be Jehovah. Cut this "we don't identify the designer" crap. Sell that to some sucker who might fall for it. It was designed by Christian radicals (notably, the lawyer, Johnson) with a strategy to wedge primitive biblical literalism into school curricula under the guise of science, as a way of violating the Constitution without paying the penalty for doing so. Now, it's been framed in such a way that some believers in Yahweh and Allah have been suckered in, but that's because they, too, favor violating the Constitution in this manner.
2. But even if we ignore that, by it's very definition, it says that certain unnamed and never identified (and thus, conveniently untestable) "features" of the universe are unexplainable by natural causes. Thus, by definition, these features are explainable by non-natural causes. "Supernatural" means not existing in nature or subject to explanation according to natural laws. As such, in its definition, intelligent design incorporates the definition of the supernatural.
3. Even if you go with a "weak" ID theory, and merely posits that the "intelligent designer" is "one possible explanation" or is "the better explanation" than natural causes, you are still posting a cause that is external to the natural world, and is, therefore, untestable and therefore "supernatural." And once you open the table to untestable claims, not only are you ceasing to do science, but you are ceasing to be able to make qualitative judgments like, "better explanation," scientifically speaking. Without being able to test, there is no scientific "better."
4. Even if you consider the even more weak definition of intelligent design, which encompasses panspermia and similar extraterrestrial origins for these unnamed and unidentified "features," the inclusion of them merely masks the problem and puts it off by one step. First, some of these sub-theories could conceivably be scientific, but that does not make ID as a whole scientific, anymore than putting a drop of ink in a glass of milk turns the milk into ink. Realistically, however, no one who is into ID does it because he is a panspermia theorist. They do it because they are god-folk. Finally, the inclusion of an alien "intelligent designer" only puts the problem off one step. ID speaks to the ultimate origins of these unidentified features. Saying that they were created by space aliens only dealys the problem, because it begs the question of the the origins of those aliens.
That is why ID is inherently supernatural and thus unscientific.