It's not scientific because it's not falsifiable. It could be falsifiable if some characteristics of the designer were hypothesized, but this is scrupulously avoided, mainly because the whole motivation for the ID movement has nothing to do with science, but rather with inserting itself into public schools, which it couldn't do if the designer's characteristics were specified. However, if ID'ers were truly interested in formulating a testable hypothesis, they could hypothesize that certain features of life are designed, and if that's the case, then the designer could not or would not do X, where X can be any potential observation. If X were actually observed, then ID would be shown to be false, or at least that particular ID hypothesis would.
The reason that ID in general cannot be testable, however, is that for ID to be held seriously, X must be something that is not observed. Therefore, ID must be found to be consistent with all known observations. However, evolution is also consistent with all known observations. Therefore, ID and evolution would predict the same things, at least with regard to known observations. If ID'ers want to go out on a limb and predict something that evolution doesn't, then maybe ID can be taken seriously as science. That seems pretty unlikely, however. I will admit that, in part, ID'ers now might have a testable hypothesis, but it's not the one that they want. ID'ers, as far as I can tell, seem to make several related, but distinct claims. Namely:
1. There exists some feature in biological systems. This claim is obviously testable, and in most cases is not disputed by non-ID'ers.
2. The feature referred to in claim 1 could not possibly have formed via evolution. This is also a testable claim. To falsify it, an evolutionary mechanism must be put forth. I have yet to hear a claim of this type that hasn't actually been falsified.
3. The first two claims taken together imply that life must have been designed. This is the claim that is untestable. It's possible, after all, that there is some other natural process other than evolution that could have produced the feature in question without the need for a designer. You would have to find an observation that conclusively proves that some natural explanation is correct in order to falsify this claim. Conclusive proof of the truth of an explanation is impossible and not something that science ever claims. Therefore, falsifying this claim is not possible.
The same objection could be raised to the theory that an object you find on the ground was intelligently designed. How would you falsify it?
The same could be true if one of the Mars rovers found evidence that could be traces of life but also could have been formed by an unknown natural process. In such a case, would science prohibit NASA researchers from creating a hypothesis about the presence of life in the past on Mars, simply because we may never be able to confirm or falsify that the evidence in question was the result of life or a natural process? Similarly, paleontologists develop all sorts of theories about past life form based on scant evidence that may never be enhanced or improved, and thus never be confirmed or falsified. Does science prohibit this sort of speculation?
The only characteristic that really matters for ID is that the designer was intelligent and parts of the design were deliberate. The evidence that would prove it are features that could not have been produced by natural selection. The falsification involves showing that those features did, in fact, arise through natural processes. Where the data is inconclusive, one could speculate either way. But that sort of thing is hardly uncommon in science.
but this is scrupulously avoided, mainly because the whole motivation for the ID movement has nothing to do with science, but rather with inserting itself into public schools, which it couldn't do if the designer's characteristics were specified.
This suggests that your problem is with the messenger, not the message. The motivations of the advocates of ID are entirely independent of whether ID is legitimate science or not.
However, if ID'ers were truly interested in formulating a testable hypothesis, they could hypothesize that certain features of life are designed, and if that's the case, then the designer could not or would not do X, where X can be any potential observation. If X were actually observed, then ID would be shown to be false, or at least that particular ID hypothesis would.
The hypothesis is that the designer would incorporate features that could not arise simply through natural selection or some other natural process. The falsification of any particular feature as proof of ID is to demonstrate how it did arise through a natural process, which is, in fact, what some evolutionists are doing who are actually engaging the idea that features exist which could not have evolved rather than wranging with the motivations of the advocates.
The reason that ID in general cannot be testable, however, is that for ID to be held seriously, X must be something that is not observed.
What would falsify evolution?
Therefore, ID must be found to be consistent with all known observations. However, evolution is also consistent with all known observations.
Actually, several ID advocates argue that it isn't. Specifically, the whole idea of irreducable complexity -- that is, a complex system that could not have been the result of natural selection -- is based on the idea that evolution is not consistent with all known observations.
Therefore, ID and evolution would predict the same things, at least with regard to known observations. If ID'ers want to go out on a limb and predict something that evolution doesn't, then maybe ID can be taken seriously as science.
ID is predicting something that evolution doesn't -- that biological systems may contain features that cannot be explained by evolution. In many ways, the prediction of ID is that evolution is falsifiable by looking for features that evolution cannot explain.
That seems pretty unlikely, however. I will admit that, in part, ID'ers now might have a testable hypothesis, but it's not the one that they want. ID'ers, as far as I can tell, seem to make several related, but distinct claims. Namely:
Again, you are confusing the message and the messanger. That's entirely irrelevant to how legitimate ID is as a theory.
1. There exists some feature in biological systems. This claim is obviously testable, and in most cases is not disputed by non-ID'ers.
2. The feature referred to in claim 1 could not possibly have formed via evolution. This is also a testable claim. To falsify it, an evolutionary mechanism must be put forth. I have yet to hear a claim of this type that hasn't actually been falsified.
Some of the falsifications are weak while others are persuasive. Some of them (such as the eye lens example posted her today) are very speculative and declare victory simply because a possiblity can be imagined, which really hinges on a leap of faith as much as evidence. But, yes, that's the general mechanism at work here.
3. The first two claims taken together imply that life must have been designed. This is the claim that is untestable. It's possible, after all, that there is some other natural process other than evolution that could have produced the feature in question without the need for a designer.
There is a point where assuming that a natural process must exist becomes begging the question. It's possible, after all, that reality is simply an illusion and that we are all brains in a vat on an alien spaceship. That does not make it probable or a reasonable theory to build your life around. At some point, it all winds up in Postmodern Wonderland.
You would have to find an observation that conclusively proves that some natural explanation is correct in order to falsify this claim. Conclusive proof of the truth of an explanation is impossible and not something that science ever claims. Therefore, falsifying this claim is not possible.
But aren't you also claiming that the assumption that everything is the result of a natural process is also not falsifiable?
Put another way, under what circumstances would you believe that life didn't arise and develop through natural processes, alone?