Intelligent Design, in general, is a hypothesis that suggests that (a) God tinkered with creation here and there, fixing this a bit, updating that a bit because evolution (supposedly) doesn't cover certain advances in biological structures. Critics call it "creationism in a cheap tuxedo" because the primary supporters are from fundementalist Christian groups who want creationism in the classroom. Some of the supporting agencies are young-earth groups who teach a 10,000 year old universe with dinosaurs on the ark, etc. Others simply don't like the suggestion that evolution is a supported science along with plate techtonics and electromagnetic theory.
From a scientific standpoint it fails because it doesn't predict anything, have any laws or defined methods. The few bits of science it does have tend toward big-number statistics that attempt to determine what's been designed and what hasn't. There is no "theory" to compare with evolution, which has an enormous amount of scientific work supporting it. At best its a hypothesis, although a poorly defined one.
From a theological standpoint, it fails because it can suggest an incompentent or evil designer. Since it never names the designer (intentionally) so as not to appear overtly religious, it invites ridicule as biologists point out bits of nature that seem badly "designed" or which are horrific. For example, some of the abilities of parasites and insects are pretty nasty in their complexity and gruesome purpose. Also, it allows the designer to be an alien (which I suppose thrills the Raleans and Scientologists), a demon,a devil, etc.
One rallying cry is "teach the controversy", which begs the question of their being a controversy to teach. By redefining what we are teaching in science class, you invite legitimizing all sorts of lunacy. How about Holocaust Denial in history class? There's a number of people who claim that there was no genocide in WWII, or deny any number of other historical events. Were the pyramids built by aliens or were they perhaps giant water pumps (not kidding, search the internet)? Teach the controversy!
