These are very dangerous ideas. The people who hold them are often good people - moral, well-meaning people who probably reason that God, being infinitely good, would never will anything bad. From that perspective this idea makes sense, but let's look deeper:
1) God's will makes an action moral (and morally compulsory), no matter how awful we might otherwise think the action to be. This is not a hypothetical point. All of the Abrahamic religions (Judaism, Christianity, Islam) descend from Abraham. He was ordered by God to murder his child, and was going to do it, and in the Abrahamic faiths this is considered a righteous thing. In this story, Abraham was stopped from committing the deed, but the message of the tale is still clear: When God orders you to commit an atrocity, you do it. No questions asked.
2) In the minds of believers who subscribe to this, God willing an act either makes that act intrinsically moral or overrides all other moral considerations (depending on how exactly they view morality. For some there is no belief in "other moral considerations" to conflict with God's will in the first place because "do God's will" is the ultimate source of moral law.)
3) This can, and does, lead people to commit acts which they would otherwise condemn as immoral.
4) In cases where someone is thoroughly convinced that they are doing God's will, there is essentially nothing that can convince them that what they are doing is wrong. No argument can be made against this; God's will trumps all.
5) We find, even if you accept that God is real and has a will regarding human behavior, that people are extremely inconsistent in their understanding of what God's will is. God's will really only comes to people through 3 sources: divine text, spiritual leadership, and personal revelation. None of these provides any means of determining God's will which can be generally agreed upon by believers. Not only have the followers of Abraham split into 3 major religions, but within each of those religions there are myriad sects, each with their own particular interpretation.
For Christians, Christ brought a new message of love and fellowship to the religion, but also said "Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them." These messages seem to conflict, and Christianity is rife with disagreement over what is or is not still the law in their post-Christ version of the Abrahamic faith.
On homosexuality, for example, Christianity varies widely. Some think that God's will is that homosexuals should be killed, and the Old Testament directly supports this. Others believe that homosexuality is a sin like any other, but that homosexuals themselves are still to be loved. Still others fully support and embrace homosexuals. A similar variety of opinion exists within Judaism, and indeed across world religions. Reasonable people of faith acknowledge that "sincere Christians can legitimately reach different conclusions on particular issues in good faith." So what then, is the will of God? It's a question that either no one can answer, or that everyone can answer and believe that they have the one true interpretation of God's will. As moral systems go, that is about as subjective as you can get.
6) According to the Old Testament, God has willed some things that almost any of us would find atrocious. This creates a situation where any command from God is plausible, because literally nothing would be out of line with what God has willed in the past. He did destroy the whole world after all, and has promised to do it again. If you accept that as a godly and good action, then nothing is off the table. Even the bringing of (nuclear?) Armageddon then becomes not a question of morality, but of timing. I do not suggest that most religious people would come to this conclusion, only that the conclusion is available to them and consistent with the doctrine of "God's will." (And surely, on a planet with 6 billion people, there are more than a handful that are truly this crazed.)
7) All of these together create a belief structure where it is possible for a person, in good faith, to believe that it is not only righteous to commit an atrocity, but morally required of them. History is rife with examples of this, and I will not repeat them all here. Suffice to say, if you looked into the heart of Al-Qaeda, or the Army of God, or the IRA, you will find people who sincerely believe that they are doing God's will. The same is no doubt true of Christian terrorists, Jewish terrorists, Muslim terrorists, and religious violence in general. God's will, God's will, God's will, and God's will.
That is the power and danger of this idea.
At the core of it all is the simple notion that normal human morality can be trumped, that there are circumstances in which it would be ok to murder your child. It is similar in this respect to the secular notion that "the ends justify the means." Both of these assert that good can be accomplished through acts that we would otherwise consider to be evil. Even though I believe that "the ends justify the means" is a very dangerous idea, I also acknowledge that it can at times be true. We can, in these circumstances, weigh the good that we expect, the evil to be done, and look to whether any net good is done. The real danger in that case is that the good result is often promised but seldom realized, whereas the evil done to achieve it is immediate and irrevocable. Almost all evil is performed in this fashion - villains do not wake in the morning, twist their mustaches, and set out to be as awful as possible. Even Hitler believed that what he was doing was morally justified, and so, we must be careful of our moral systems and how we allow ourselves to justify our actions. Any action justified with "the ends justify the means" should be considered thoroughly and with suspicion.
Not only does the doctrine of "God's will" allow us to trump normal human morality, but it does so in a fashion that is so vague, so subject to personal interpretation, that anyone can take any position whatsoever, claim it to be the will of God, and stubbornly believe that. Indeed, this is precisely what we see in the world and in history. The fact that most religious people will never go on a killing rampage for God does not excuse them for supporting a moral structure that
"God's will" as an entire moral system is an idea that is morally bankrupt, both in theory and in practice. The key point here is not that religious people are immoral; that is obviously not true. The point is that this moral structure is wrong, and that those moral people who subscribe to the doctrine of "God's will" are moral despite this doctrine, and not because of it.
We simply must keep ourselves in check with a moral system that has objectively understandable benefits to ourselves and to others. Humanism is one such system. There are others, each concerned with the effects of our actions on ourselves and others. This is the foundation of morality, and this is why killing your child is not moral, even if you believe with all of your heart that God wills it. A moral belief structure that cannot make this distinction is a gateway to justifying evil, and history bears this out. A path of actual morality does not necessarily demand that you abandon your faith, or God, or your religion, but it does require that the doctrine of "God's will" be put aside.