I will start by saying why it is extremely improbable that the Christian god exists.
I once saw a great debate at Purdue university on this topic and the debater who described which is more likely worded it almost perfectly. If anyone can find this video or knows where it is please post it as it's a much better explanation than I'm about to provide. I'm not a philosopher or an English major, but I'll do my best. He used an analogy of a cake. It's your birthday and you're expecting a cake. So you come home expecting to see the ingredients, the dirty dishes, and the smell of the cake but you find that none of these things are there. You can then healthily infer that no cake was made (at least in that house).
First we have to examine what a world WITH a loving (the christian god is loving) god would look like. In almost every aspect in which we look, we find that the world comes up short of our expectations. We will assume that this god cares about his creations. We will assume that the teachings in the Bible are true. You can start by looking at the pointless suffering in the world. Some christians will say that the devil is responsible, but are you saying that god allows the devil to run rampant, torturing and killing his creations at a whim? This is a christian cop-out. Some will say only the people who deserve the pain are getting it. I volunteer at a hospice for which terminally ill children stay. Are you telling me that they deserve to suffer? So we find that pointless suffering of his creations shows that he either does not exist or does not care about the fate of us.
The existence of "reasonable" atheism is my next reason. It is possible for an atheist to be logical and reasonable and to logically and reasonably debate whether god exists or not. Would it be difficult for god to provide a small amount of evidence that would substantiate his existence and allow for all reasonable people to believe in him?
The success of science without taking god into account. Throughout history you notice that religion has shrunk further and further as science and knowledge has grown. For example, we used to think rain and fire were both divine miracles. Now we know the science behind both of these. We used to think the Earth was no more than 6,000 years old (though I admit some people legitimately still believe this). Now science tells us it is not. While science can never prove that god does not exist, it has been extremely successful without taking god into account at all.
I have indeed examined the concept of "intelligent design" and "irreducible complexity". In reading a lot of what the "Discovery Institute" has published and attempted to get published into scientific journals, I've found one common theme: they all lack or are willing to ignore the abundance of flaws in human evolution. Though we have things on our body that are remarkable, it's not logical to use that as evidence for design. In fact, when we observe what we know about evolution, it's not hard to visualize how these things could have come into being naturally with the amount of time they've had. When you imagine how long life on Earth was microscopic it allows you to put things into perspective. Organisms were not allowed by the atmosphere to get much larger than a few micrometers until the oxidation event ~2.4 billion years ago. Oxygen did two things: it allowed greater diversification/size and it formed a protective shield by reacting with UV rays from the sun (We call O3, the ozone layer). Shortly after this there was an explosion of different forms and themes. More species went extinct throughout history than remained, which fits the idea of "natural selection" better than it does the idea of "theism". Humans have been slowly evolving for billions of years. In that time, isn't it likely that most of our remarkable features were able to become as complex as they are now? We can also look at the flaws in our own evolution to find that evolution is very inefficient as a process. For example, the blindspot in the eye, the trachea and esophagus positioning in our throats cause us to choke easily (ie: "it went down the wrong pipe"), the appendix, etc.
Discrepancies in the Bible and Qur'an is more specific to the Abrahamic god. Lets face it, and even most christians will have to admit this, most of the stories in the Bible are too fantastic to have any scientific possibility whatsoever. Isn't is wondrous that they only occurred at that time and not any more for us to see? Isn't it wondrous that science has found out so much and the Bible's validity/stories have become so less true over the last hundred years?
Common beliefs and themes in both the Qur'an and Bible suggest that all non-believers will go to hell to suffer eternally. I bring us full circle now back to my original post in which god "cares" about his creations. I understand the need for punishment. However, given that god has provided no evidence of his existence, does he really believe eternally damnation for nonbelievers, gays, and believers of other deities is a just punishment? Many christians will say this isn't true, and to them I say I will post the quotes from the Bible itself to verify if need be. I'm not willing to accept that a god whom loves/cares about his creations would put forth such a sentence.
Anyways, that's most of what I have to say on the subject.