Robin Parry is an evangelical writer and theologian whose books include The Biblical Cosmos: A Pilgrim’s Guide to the Weird and Wonderful World of the Bible, which is an excellent overview of ancient cosmology in the Bible, and how to understand it as a modern Christian.
The problem with Intelligent Design (ID) is its tendency to look for God (or simply a “designer”) in the gaps of scientific explanations. So-called irreducible complexity, for instance, is seen as evidence of this “designer” because science cannot (in principle, we are told) explain it in terms of natural processes. But if future science did actually explain any alleged instances of irreducible complexity, then such instances would cease to be evidence of the “designer.”
The problem here is that the “designer”—which almost every ID advocate thinks is the biblical God—is pictured as one being among others (albeit a more intelligent and powerful one) acting as a cause in the world in the same manner as other causes act in the world.
The reason that this is a problem, at least for Christians, is that classical theology does not picture God in this manner—as one cause or being among and alongside others. Rather, divine Being is of a fundamentally different kind from creaturely being, and divine causation acts at a different level altogether. God is the one who imparts be-ing to the whole of created reality and who enables all of the powers of causation within creation. So God was the explanation for the whole, but was not to be found in the gaps.
The explanations of the empirical sciences function at the level of secondary causation within the created order, and pay no attention to metaphysical questions of primary causation. As such, God does not feature in scientific explanations. This is unproblematic so long as scientists don't imagine that reality can be encompassed within the realm of what science can explain—that road ends up collapsing in on itself. Treating some things in the world (but not others) as the result of God rather than of inner-creational causes is to mix up these different levels of explanation. Setting divine and creational causes up in opposition as some kind of zero sum game is unhelpful.
Furthermore, the most that ID could ever demonstrate is that certain things in the world (but not the the world as a whole) were designed by a very intelligent (though not omni-intelligent) and powerful (though not all-powerful) being (or groups of beings). But such a being is more like an archangel than God. And of such a being we may still ask, "Who designed it?" for it would certainly not be the kind of thing that could explain its own existence. This intelligent designer would be as infinitely removed from God as a flea.
I am not for one moment suggesting that those who believe in God should not look at complex systems within creation and marvel at how they manifest God's goodness and power—after all, such complex systems live and move and have their being in God, manifesting the Divine Logos—but that is a very different issue from seeking to find them as evidence of direct divine intervention. There be dragons!