My reasons for not believing the following religions:
Protestantism: I don't believe in Sola Scriptura (Scripture Alone) because that makes little sense seeing as (a) it doesn't say that that ought to be the case anywhere in the Bible, in fact in 2 Thessalonians 2:15 it says that tradition is to be valued as well; (b) the books of the New Testament were not all completed until at least the end of the first century, were not compiled until the fourth century, and were not made readily available until the fifteenth century when the printing press was invented; and (c) I think the Church should be unified as suggested in the New Testament, which would be impossible with private interpretation (there are 41,000 Christian denominations!). I don't believe in Sola Fides (Faith Alone...I'm not sure if I got the Latin right) because again, that cannot result in unity which I believe is important. I have other reasons as well, but I don't have time to go into depth right now.
Mormonism: The lack of archaeological evidence of the events found in the Book of Mormon is suspicious, but otherwise, I have a lot of respect for Mormons and their religion and those I have met (including d_e ;) ) have given me a good impression of them overall. The polygamous Mormon sects are a different case though.
Judaism: This one is fairly obvious. I obviously believe the Messiah has already come and believe he dispensed with the fine details of the Mosaic Law and gave the next step up.
Islam: Despite what many insist, Islam is certainly not a religion of peace and this is apparent from the Qur'an. I believe in peace (though I think war is necessary sometimes), thus I cannot maintain the ideals of this religion.
Hinduism: I'm not going to even bother explaining. The same goes for the ancient Greek, Babylonian, Roman, Egyptian, etc. gods and mythology.
Atheism: The relative subjectivity of atheist's morals doesn't attract me very much. I believe that right and wrong are fixed and do not evolve over time. Atheists believe so many different things and my reasons may not apply to all of them, but I am also opposed to moral subjectivity because of all the potential problems that can spring from it; the certainty of the nonexistence of any god seems no more reasonable than the belief in such a god and atheism doesn't explain so many things (science may one day, but I doubt it); atheists (this is not necessarily a problem with atheism itself) frequently try to impose their "superior intellect" on us primitive religious, a trait I find greatly detracts from their cause, as I think humility is a great virtue that would solve so many problems if more people possessed it; and I cannot agree with the ideals and ideas held by many atheists since they conflict with many ideals and ideas that are positive and beneficial in my personal experience and examples throughout history.
Agnosticism: Not much to say about this one. Similar to atheism, but not as bad.