Intelligent design fails as a scientific explanation in three ways.

1. It's not falsifiable.
2. It's a sly attempt to shift the burden of proof
3. Like all creation stories it creates more problems than it solves and fails the test of parsimony

1. A scientific theory must be vulnerable to the data. This is fundamental. It means that at least in principle, there is some experiment one could do whose outcome might disprove the the theory. Simple example would be the theory that all cats are black - finding a single nonblack cat disproves this theory. What test could possibly disprove the theory of Intelligent Design?

2. "Irreducible complexity" boils down to asserting that since I cant think of another explanation it must be intelligent design. The person who takes this position can never say "there IS no other explanation" because this is almost impossible - like proving that every cat is black - when do you know that you have checked them all?

3. A good theory SIMPLIFIES not complicates. Intelligent design raises the obvious: who is the designer? why does he design? who made him? why was he made? who made the designers maker and why? .... In fact you now know less about the origins of life than before you adopted this theory.