I don't mean to lower all theists to the level of Ray Comfort, but his well-known argument reflects the same basic principle that all creationists and ID advocates use- "Every building has a builder, every painting has a painter." Comfort says that the appearance of design is indicative of intelligence; that complexity organized in such a way that it operates toward a specific purpose or purposes must have a sentient designer behind it, much like we design buildings or machines for a specific purpose. Is this argument valid? All examples we have of design in our everyday life exhibit complexity and operate in a particular manner, towards a particular goal.
The largest objection to this reasoning was best articulated by Matt Dillahunty of The Atheist Experience, so I must give him the credit- "We don't know what an undesigned universe looks like." This sums it up very well. We do know that design generally shows complexity and features a discernible function; however, we may not have thought hard enough about what nature will do when left to its own devices. It is likely that a universe so complex, with such a complex fundamental nature, will give rise not only to complexity but to such phenomena as life. Indeed, I believe that a universe with no surprises, no oddities and no emergent phenomena would be much more confounding than the universe we see around us. The elements of both chaos and order, complexity and entropy are all around us. Our creations are complex, yes- but the entire universe is very complex, so that rather rules out the option of electing complexity as remotely compelling proof for "design".
Now, there is another variation on ID which takes yet another step back from life on Earth and instead turns to the laws of nature themselves. This is the "fine-tuning argument", which argues that surely the six universal constants must be evidence of a designer, for if any one of them were a fraction of a percent lower or higher, life as we know it would not exist- in fact, such necessities as the forming of stars would be impossible (I'm no scientist, but you get the idea). Therefore, a supervising being must have "twiddled the knobs", perfecting the conditions for life. This fails in two ways; one is the misunderstanding of what the "laws of nature" actually are, and one is, we might say, a failure of imagination.
The laws of nature are not tangible laws. In reality, they do not even exist outside of the conceptual. They are descriptions of the universe's behavior. These laws are not discovered; they are invented. They are our interpretation of the fundamental nature of our universe. Take the color blue- it's not really "blue" in a deep sense of the word. The universe neither knows nor cares about the concept of "blue". It's a description that we use to refer to a perception. Now, these laws are not conceptual in the sense that they do not affect everyday life- if one jumps off a building, he will fall whether he believes in the law of gravity or not. But the law of gravity is not a tangible, natural entity that governs the universe. The law conforms to the universe, not the other way around.
The second flaw I find much more interesting. Pay close attention to this argument- "If the laws of nature were even slightly different, life/the universe as we know it would not exist!" Well, this is a rather meaningless statement. It is doubtless that the conclusion is correct, but why does that matter? Let's imagine that the universe's laws were slightly different (although in reality they couldn't be- this is the universe's fundamental nature; if that nature was different, it wouldn't be the universe). As I explained above, when we deal with this much space, this much time, and this much complexity in nature, there will be surprises left and right no matter which numbers we're feeding into it. Doubtless there would be phenomena just as- if not more- awe-inspiring than life or black holes (a fascination of mine). Now, you may rightly claim that I have absolutely no empirical evidence for that statement. However, if we are to trust the power of logic, then due to the frequency of phenomena in this universe alone, under all manner of conditions, and at every level of physical interaction, we can reasonably say that phenomena spring up rather frequently given such an astonishing complexity of nature. Now, if you still disagreed with that on the grounds that this universe can have no say on the likelihood of emergent phenomena in other conceptual universes, then I have no retort- but you've just admitted that the "fine-tuning" argument is bogus, so there we are.