While my criticisms in the other thread were indeed mostly centered around immoral parts of the Bible, that is a relatively small part of what drives my overall opinion about religion. And, my criticisms of religion are fairly universal (Christian/Islam/Scientology/Mormonism/YouNameIt)--I speak mainly about Christianity because it's what I know best and was raised in, and living in America it's obviously the predominate faith.
To me the unavoidable nail in the coffin of religion is the Problem of Evil. Essentially the logical dilemma is it's impossible for disease/suffering/etc. to exist in a world whose sole creator is all-powerful, all-knowing AND all-good. To me this means there is no all-knowing, all-powerful, all-good god.
Consider a scientist in a lab who has the ability to clone a human baby, but knows for a fact that in so doing, the cloned human baby will live for a mere 12 months, in intense daily agony, and then will end up dying a horribly painful death on his first birthday. Would it be criminally and disastrously immoral and evil for this scientist to create said baby? Of course. Yet "god" does this type of thing many times every day. So I now know that God can be HIGHLY immoral and at times even evil. Nobody taught me that in Bible school, yet as a rational adult now this line of logic is inescapable.
With that in mind let's circle back to the immoral parts of the Bible (such as killing gays), and all of a sudden, it's pretty easy to see where these immoral parts might actually fit into God's precepts and worldview, because we can see similarly evil acts happening every day all around us in the world, in a world that was reportedly entirely created by God. So who's to say God didn't directly inspire a biblical author to talk about killing homosexuals? After all God created the AIDS virus, which senselessly killed off millions of mostly gay people. Naturally I don't believe any of this, but the Problem of Evil leaves the door wide open to all of this--to sum it up, if God can be evil in everyday life and in nature, then we just don't know where else God may be evil, including in his direct commands to his followers.
Most importantly, I just find it to be a fairly absurd scenario when viewed from 30,000-feet:
at least 9 billion years after creating the universe, an all-powerful being then creates one planet out of hundreds of billions of planets in the universe, then on that planet over more billions of years have plants and animals evolve from tiny cells, then eventually having a small subset of primate animals evolve into humans, then having humans continue to live their lives in many ways as they did when they were more primitive animals e.g. still having random sex and stealing and so forth, then apparently God at some point determines that sex/stealing/etc. is "wrong" (of course God created sex/stealing/etc in the first place...so this is already starting to go off the rails), then he tells some of these humans to tell all the other humans this (e.g. 10 commandments), then God decides to "save" all of these humans from their sex/stealing/etc. (wait, why do they need "saved" again???) by creating a semi-human/semi-divine son with the full intent that his son will endure torture and literal human sacrifice (which is another example of God being immoral--he could easily "save" humans without human sacrifice if he wanted), then after his son goes through all of this, God doesn't even ensure that humans properly document everything that happened to his son in such a way that future humans would even be compelled to believe any of it, given the highly questionable authorship of the documentation over the centuries, and given the thousands of other resurrections and miracles that have also been reported (through word of mouth of course) in all parts of the world, etc. etc. etc.