I finished the Koran a few days ago, and here are some reactions. Since the book is so big, I can't really document every claim that I make with a specific reference, so please don't put too much stock in what I have to say!
1. The God of the Koran is not as emotional as the God of the Old Testament, and its author does not rhapsodize about the love of God like the apostle Paul. He appears to be a cool, detached judge who wants people to do the right thing. I can't say that the Koran lacks a God of love because it holds that people should trust him, and you can't really trust a deity who is not good and loving. Moreover, like Judaism, the Koran appears to advocate a just society, one in which men take care of their divorced wives and help the poor and the vulnerable. I can respect and fear the God of the Koran, but can I love him? I find that difficult, especially since he doesn't passionately proclaim his love for me.
2. The Koran is really big on the resurrection from the dead, such that it becomes its main topic near the end of the book. Apparently, a lot of people in Muhammad's day (even the Jews the Koran criticizes) did not believe in the resurrection from the dead. But such a concept is important in the Koran, since one needs to be raised from the dead in order to be judged in the afterlife. And God as judge is perhaps the most salient theme in the book. The Koran rhapsodizes a lot about God's power in creating the heavens, the earth, and human beings, but its aim in doing so is usually to convey a message of "See, God is powerful, so why do you say he can't raise the dead?" I like to celebrate God's majesty as creator, but it's usually a bummer when that idea leads to God as a judge. I'd much prefer it to lead to God as love, or God as glorious, or God as lover of beauty.
3. I eventually got to the point where I didn't meditate on my daily reading because it was the same thing that I'd read before: God can raise the dead, so he will judge you in the afterlife. I'm sure that religious people are gasping at this statement, for the last judgment is a very serious thing. Who am I to trivialize it as if I'm merely reviewing a book or a movie? That's a legitimate criticism, and I should remember to take seriously the religions that I study. Still, while I acknowledge that judgment is good because there are unjust people in the world who need to be punished, I have problems with a God who scrutinizes everything I do. I think I arrived at the point where I acknowledged that I'm just not perfect, and I don't beat up on myself every day on account of that. At the same time, the Koran's path to salvation looks fairly manageable: do your devotions to God and help other people. Then, at the last judgment, God will weigh your good deeds and your bad deeds (as in Judaism) to determine if you go to a nice, pleasant afterlife (with good drinks and virgins, though I didn't see the number seventy) or an ever-burning hell (which is paradoxically hot and cold).
4. God sends prophets to turn people from sin, but I can tell that Muhammad got pretty frustrated with people's rejection of his message. At some points, God tells him to preach the word and not worry about other people's reactions, something we also encounter in Jeremiah and Ezekiel. Near the end of the book, there's a recognition by Muhammad that he really can't change the hardened sinner. And the subject of hardening appears throughout the Koran, which affirms that those who reject God's message are only setting themselves up for further spiritual darkness. It's kind of like many Christians' portrayal of the Pharaoh of the Exodus (whom the Koran also sites as an example): he hardened his own heart by rejecting God's message, so God sealed his hard heart and made it a permanent condition. That doesn't entirely resonate with me, since I prefer a God who never gives up on anyone. But we see that sort of idea in the Bible (Isaiah 6; Jesus' parables obscuring the truth; Hebrews' statements about Christians who turn their back on Christ).
At the same time, when God hardens certain people's hearts in the Bible, he often seems to have a beneficent end in mind. God hardened Israel and thereby enabled her destruction and exile, but maybe she needed that turmoil to become spiritually pure. Sure, she could have offered God a shallow and superficial apology accompanied by a short-lived reformation, as she did during the time of Josiah, but what good would that have done? She may have needed to be left in her sins so she could receive chastisement and become more spiritually fit for God's purposes. Moreover, in Romans 11, Paul says that Israel's temporary hardening will lead to the salvation of the Gentiles and ultimately herself.
5. The Koran's interaction with the Bible and rabbinic traditions is interesting (to say the least). At some points, there is significant overlap, as when the Koran refers to God's tests of Abraham (a midrashic theme) and makes statements that evoke Jesus' parable of Lazarus and Dives (Luke 16:19ff.). At other times, its knowledge of the Bible appears rather skewed or indirect, as when it places Haman in the time of the Exodus. A colleague of mine speculates that Muhammad heard these stories from travelling caravans, which sometimes conveyed the Bible and rabbinic stories accurately, and sometimes did not.
Muslims who believe the Koran is inerrant have their answers, as Christian fundamentalists do for those who talk about the "errors" in the Bible. Some say that the Haman of the Exodus was not the same as the Haman in Esther, and others assert that the Book of Esther is just plain wrong about when it places Haman.
6. Like Romans 1, the Koran maintains that God's creation and sustenance of the cosmos is evidence that he is the one true God. Those who reject Allah by worshipping idols, therefore, are rejecting God's clear signs. Muhammad was probably speaking to people who believed in Allah and saw him as the creator, but they also worshipped idols.
But there are other signs: Muslim victory in battle, the Koran, etc. At one point, the author asks people to produce a sura if they think the Koran is simply of human origin. In these days, I don't think that would be too hard, since there are a lot of literate people who can produce beautiful compositions. But was that as true back then? I don't know. The author of the Koran made that argument for some reason!
Was God involved in the foundation of Islam? I don't know. Near the end of the book, there's a story about how certain warriors were uniting to destroy the Caba, and God sent down birds to defeat them (or so said the footnote). The Muslims had a miraculous victory! Should I give that account any credence? On one hand, it is probably proximate to the time of the battle, so I have a problem blowing it off completely, since people may not write something that is so blatantly untrue to their contemporaries. On the other hand, there are also medieval stories with miraculous elements, which historians don't take all that seriously.
7. There are several chapters on war, which is presented as self-defense, or as the spread of social justice. The Koran really stigmatizes Muslims who don't go out to fight God's battles. We also see that sort of thing in the Bible, as when the Song of Deborah criticizes those who didn't fight with their fellow Israelites (Judges 5). At the same time, Deuteronomy 20:8 allows those who are afraid to sit out of the war. In this case, the Bible seems to be more understanding of human foibles than the Koran. Usually, it's the other way around, since the New Testament strictly prohibits divorce, whereas the Koran allows it, provided the man take care of his ex-wife.
8. The Koran's depiction of Satan is interesting. According to the Koran, Satan was an angel named Iblis, who refused to bow down to Adam when God commanded all the angels to do so. A Harvard colleague of mine said a while back that Iblis was passionate for the glory of God and didn't want to compromise that by worshipping a human being. But that's not what I see in the Koran. Rather, Iblis thinks he (Iblis) is superior to Adam, since he is made of fire, whereas Adam is made of dirt. After his fall, Iblis aims to be a stumbling-block in the path of human beings. While people may worship Satan and his jinns in the here-and-now, the Koran maintains, these spirit beings will act like they don't even know their worshippers at the last judgment. At the last judgment, it will be every man (and spirit) for himself, so we'd might as well trust God and not evil spirits who will ditch us!
9. I didn't find a lot of wisdom in the Koran: how to live a successful, happy, and righteous life and overcome your sinful passions. I encounter more of this in other writings, since the Bible has wisdom literature, and rabbinic midrashim present Torah study as a cure for man's sinfulness. But the Koran doesn't seem to do this as much, for its message is "Be righteous, or else." It tells you to go somewhere, but it doesn't offer much guidance on how to get there. But I could be wrong on this, since more readings of the same book can easily produce a different impression.