For those who hold that there is some very close relationship between God's will and what's right or wrong, there are two ways of specifying what this relationship is. Consider an action X that we'd all agree is wrong. A theistic basis for morality would go as either of the following:
X is wrong because God condemns X.
God condemns X because X is wrong.
Really the only plausible choice between the two is (2), and while this can maintain divine commands as crucial epistemological aids in coming to know what is right and what is wrong, and as important motivational aids to developing the strength to act morally, it dooms the divine command theory as a logical (or theoretical or conceptual) basis for right and wrong.
To see this, let's look at what would happen if we opted for (1). We'd have to say that, e.g., torturing children is wrong because God condemns it. That is, all that makes it wrong is God's condemning it. This seems to make matters of morality wholly arbitrary (arbitrary in just the way most people who hold theologically based ethics are trying to avoid), for if God had decided to will otherwise, then torturing children would be perfectly permissible, and maybe even obligatory.
Say my father tells me it's wrong to take a cookie before dinner, and, being a pestering kid, I ask why. He tells me, "Because I said so." Dad would then be claiming (perhaps unwittingly) that the only source of the wrongness of having a cookie before dinner is his command, and that if he had decided to command differently ("Gorge yourself on Oreos before dinner or else take out the trash!"), then the right thing to do would change. The problem with this is that it seems to make what behavior children should engage in entirely arbitrary, and hence there'd be no reason (other than avoiding punishment from an arbitrary authority figure) to follow such codes. (Adolescents discover this and frustrate their parents' reasoning with objections like "Why should I do it just because you said to?")
The theologically sound reply to make, it seem to me, is to say that the situation with God is different than it is with my father. Namely, God's decrees aren't arbitrary; God couldn't will that we torture children, for torturing children is wrong and God would never will us to do anything wrong. This is the sensible reply, and it's probably even true (if God exists and is interested in morality). But note that it abandons (1), for in the above italicized clause the wrongness of torturing children is appealed to independently of God's decrees. It is claimed that such actions are already wrong, and God condemns them because of this antecedent or independent wrongness (together with God's benevolence). All of this is to embrace (2), which would become our only plausible choice.
This is theologically sound, but realize what we have done: in choosing (2), we have denied that God's decrees are what makes right actions right and what makes wrong actions wrong. God's decrees would not be the source of morality (even though knowing God's omniscient and benevolent will would be a reliable way to know which actions are good and which are bad, and even though deriving strength of character from belief in God may be motivationally important in helping us to accomplish such things as avoiding temptation). Something may be wrong, and God may condemn it, but it can't be wrong because God condemns it. So we have to provide some other basis for morality--Aristotelian, contractarian, utilitarian, Kantian, or something else.