I cannot speak for Christianity, but Islamic metaethics is far more complex, historically and theologically than made out in both the blog-post and the above post. The following will of necessity be quite long.
It appears early Mu'tazilite doctrine coincides with Christianity - that good and evil are objective, almost mathematical constructs, and importantly (for the Mu'tazilites), rational ('aqli). Mu'tazilism was a diffuse rationalistic "school" in Basra and Baghdad that gained prominence in the 8th and 9th century (especially when it became state doctrine of the Abbasids under Ma'mun and Mutawakkil). Mu'tazilism eventually died in the 10th century but its ideas were incorporated in Shiism about the same time and the objective nature of good and evil lives on in the Shiite school till today. In that sense, Shiism is in fact similar to Christianity; as an aside, Shiism is similar to Christianity (in particular, Roman Catholicism) in many other regards too (e.g. the apostolic successorship of Imams, the redemptive nature of Husayn's sacrifice at Karbala). God then is also, in effect, restricted by the realities of good (husn) and evil (qubh). As a corollary, Mu'tazilites established the doctrine of the mechanical nature of reward and punishment, that good will produce reward and bad punishment. The idea of an "economy of salvation", thus, seems to be present in Mu'tazilism too. The theological difficulty with this manner of thinking is, of course, with grace - how can mercy or grace be reconciled with a mechanically absolutely just God? Christians answer this with Christ and Mu'tazilites/Shiites with human works (so there isn't any real grace in pure Mu'tazilism, although it does exist in later revised versions). As a corollary of this view, Mu'tazilis developed a doctrine of absolute freewill and tawallud (that cause and effect are intrinsic properties of the world created by God - i.e. Aquinas' "secondary causes"). Qadar or God's preordainment of future events, good and bad, an early prophetic teaching, was also denied (this developed independently as "Qadarism" and was later subsumed under Mu'tazilim). Also, consequently, they denied intercession as it would contradict God's justice and the direct relationship between good and reward/bad and punishment. They also developed an interesting concept of the Optimum (aslah) where God had to create only this world because it was the best world for His creatures. God, for them, is intrinsically (dhati) good.
However, you are correct, that orthodox Sunni Islam, represented by Ash'ari theology that became dominant after the 10th century (and had its roots in the majority of early traditionalist Muslims and in scripture), had the opposite view of metaethics. It had a deep-rooted traditionalist scriptural view that God absolutely transcends human nature, and cannot be judged by our worldly ethics. However, to assert that Ash'arism advocates an impersonal God, would be very misleading. Although in Ash'arism God is ethically distant, He is metaphysically near. This is the exact opposite of Mu'tazilism and Mu'tazilites in fact accused Ash'arites of a Christian-like belief in their personalisation of God. The reason for this paradox is Ash'aris gave scripture and tradition much more value than their rationalistic intellectual rivals. This turned out to be appealing to the masses and Islamic scholarship, hence dominating Islamic religious thought for about a millennium.
Ash'arite metaethics asserts that good and evil are not objective/rational ('aqli) but are legalistic (shar'i) - that God Himself is not constrained by any morality. This, of course, requires a tremendous amount of qualifying but remains core Ash'arite (and orthodox) doctrine. Ash'arites also developed potent atomistic theories (in opposition to Hellenic hylomorphism) to demonstrate God's absolute power. They extended atomism into time, and created a sort of temporal atomism called "occasionalism" (that has some resonance with Whiteheadian process thought). God, as it were, creates every moment, and every atom some of which are incorporated into bodies (jawahir) that express attributes as accidents (a'rad). The atomistic thought of Ash'arism achieved a quasi-credal status in 11th century under men like Baqillani as it was the only idea that successfully demonstrated God's absolute power, but was later seen as only one of many interpretations although the best available physical theory explaining God's omnipotence. I think Ash'arites had a scriptural upper-hand over the Mu'tazilites, as their major comeback would be that a particular act may be considered harmful or evil but commanded by law and vice versa e.g. people may agree promiscuity is a good thing but the law makes it bad, and this is succinctly expressed in a verse: "it may happen that ye hate a thing which is good for you, and it may happen that ye love a thing which is bad for you. God knoweth, ye know not" (Qur'an 2:216)
These two ideas, an ethical subjectivism and occasionalism, do appear to be scriptural (e.g. "God is Creator of every thing" (Qur'an 6:103)). The problem was rationalising them under the influence of Mu'tazilism, the Muslim philosophers and Greek thought. The Ash'aris did this wonderfully. Although ethics are subjective, God has created in us an intrinsic capacity to see their value, called fitra (an innate nature). Fitra is conducive to a life that will be in harmony with the world as it creates the balance required; according to a prophetic tradition the fitra is also conducive to the Islamic faith. The prophet listed amongst the things of fitra such mundane things as gargling the mouth and cleaning the teeth as these are part of our humanity. Fitra is a Qur'anic theme too (see: Qur'an 30:30). In some sense, therefore, there is a "created objectivity" to ethics necessary for harmony in the world that we live in. Human evil is seen as a distancing from this faculty and a reason for corruption seen in this world (Qur'an 30:41).
Regarding, the problem of freewill, the Ash'aris devised an ingenious Qur'anic notion of kasb (acquisition) which has resonances with Stoic compatibilism. It basically states although God is the creator of our actions, we are in some way responsible for them hence acquire them - some have understood this to mean that although discernment (tamyiz), judgement, choice (ikhtiyar), intention (qasd) and consciousness itself are our own, the resulting effects and consequences ("actions") are created. God may even "seal our hearts" (e.g. 2:7) if our own decisions are bad (analogous to God hardening Pharaoh's heart in Exodus). This also gives rise to the question of evil. The Qur'an never directly attributes evil to God, e.g. in 113:2 it says evil arises from what was created, not that it was created itself. This apparent evil has been understood by theologians to either be part of a bigger cosmic plan (hence not evil at all) and/or because of human disobedience creating imbalance in the created order.
As for the absolute freedom of God and the effect this would have on a cause/effect theory of the world, the Ash'aris understood there to be a common "convention of God" (sunna of God) - which is a Qur'anic idea (e.g. see 48:23 - "you will never find a substitute for God's convention"). This means although cause and effect are, in effect, an illusion, they nonetheless remain possible constrained generalisations of nature that can be rationalised and intelligised because they are how God generally acts. This convention is important, again, because it allows harmony and balance in the created order. However, this means Ash'arite theologians are a lot more open to an objective interventionist scheme of God's acts in the world - i.e. miracles are not breaking laws because laws were never there to be broken. Occasionlism did not stunt scientific activity in Muslim civilization as claimed by Peter Hodgson because firstly, theology was never enforced under Islam (with the exception of the mihna during the 9th century) and secondly, it ignores the conventional nature of God's action.
Most importantly, I guess, is how this reflected on God's nature. For the Ash'aris, we could only know about God what God told us and we could not create limitations on God using our mind or reason or our human ethics. In other words, we know God does not lie, is just, is graceful and merciful, is loving and forgiving etc. because God has told us. This may be circular but in reality all Ash'arites have to prove is a. the Qur'an is from God and b. it is all true. They generally "proved" the first by pointing to the miracles of the prophet Muhammad and the latter they demonstrated in various ways e.g. that lies are a deficiency (naqs) which are precluded from the divine power; that God's word is eternal and one, so if part of it is true then it follows it is all true and since we know there are some objective historical truths therein it must all be true (as a single discourse cannot be both true and false simultaneously) - "who is more truthful than God in speech?" (Qur'an 4:87). A Qur'anic phrase is "were it not for the word that had passed..." indicating, not the objective nature of things, but their contingency on God. In sum, we know of God in His transcendence (tanzih) and His own description of Himself. Ash'arite speculative theology has generally been a negative theology (apophatic) although with its subsequent marriage with Sufism (despite some inherent tensions) it had an added positive element, personalising and making God immanent.
I would be doing disservice by not mentioning the other major school of discursive theology (kalam), Maturdism. Maturidis were very similar to Ash'aris and dominated in Hanafi Sunni circles, and for all purposes they were Ash'arites. In metaethics however, they sought a middle path between the objectivism of Mu'tazilites and the subjectivism of Ash'arism, with some success (it seems they are closer to Mu'tazilism). Hanbalite theologians (strict traditionalists) also share an Ash'arite metaethics.
As a pretty conservative, traditionalist and orthodox Muslim, I adhere to Sunni Ash'arite metaethics and I think it is theologically and morally more useful. Theologically, because it gets passed the grace problem without the need for an economy of salvation. Mercy and grace is the most central and dominant Islamic understanding of God (see Mercy: the Stamp of Creation by dr. Umar Farooq Abdullah,
www.nawawi.org/downloads/article1.pdf); so Islam is better understood as "the religion of mercy" and not "the religion of peace" and Christianity as "the religion of love". Mercy means overriding or replacing justice. In traditional Ash'arism God cannot be unjust because injustice is displacing or meddling in another's dominion, but since everything belongs to God He cannot be unjust. Justice, however, in the worldly sense that everyone will be paid for his or her deeds is also a legalistic (shar'i) requirement but one that is therefore flexible and not necessary (in a methematical sense). In Christianity, justice is necessary in a mechanical way, and this is why Christ's redemptive sacrifice is so important. Islam doesn't need a sacrificial lamb because Islam's God can just forgive without trespassing on divine justice (which is outside this universe) and earthly justice. The Islamic view appears to be closer to the scriptures - the Hebrew Bible, the teachings of Jesus and the Qur'an. Psalms 103 rejects the mechanical view of reward and punishment and instead introduces a merciful God that forgives without repayment. This is very similar to hadiths attributed to the prophet Muhammad. Jesus too in his parables taught forgiveness without the need for repayment. He taught the prayer "forgive us our sins as we forgive those who have sinned against us" - we forgive each other by simply letting the wrong go. The Christian God is also an immoral God because He requires payment and hence does not forgive ("a forgiveness that has to be bought by full payment of the moral debt is not in fact forgiveness at all" (John Hick)). Instead in Christianity, the criminals are let off (by God's "love") and an innocent being is made to suffer. In Islam (that is, "orthodox" Islam), both mercy and justice in the world (as conflicting properties) are shar'i (legal/scriptural) elements of God's nature (not intrinsic) so God is able to be merciful unreservedly (For this tension in Islam, see: Forgiveness and Justice by Timothy J Winter,
www.masud.co.uk/ISLAM/ahm/HadithsonJustice.pdf). As the Qur'an states "My mercy encompasses all things" (7:156), and according to the mystic-theologian ibn Arabi "all things" includes God's own anger - hence God's anger is a function of His mercy.
Finally, I think it is important to clarify the Ash'arite stance as regards to God's "morality". God is not constrained by any morality, but this simply means He is not *intrinsically* (dhati) constrained. However, there are impossibilities in God because of their contingency (mumtani' bi ghayrih), which include lying, cheating, being unjust, punishing for no reason etc. Regarding 8:43 and the vision of Muhammad before Badr that James brought up, the exegete Fakhruddin al-Razi explained the prophet was shown a part of the approaching band of the Meccan idolaters and not all of them, so he could console his companions; this was not a lie. Likewise, the substitution theory was only one of the interpretations of Qur'an 4:157, and theologians like al-Razi struggled to find moral justifications. In other words, the Muslim God is a moral and good God (in the Christian sense), not because of His very nature but because He chooses to be.
Further useful reading:
Ethical Theories in Islam, 1994, Majid Fakhry (esp. part 2 "Theological Ethics", p. 31-55)
The Alchemy of Domination? Some Ash'arite Responses to Mu'tazilite Ethics, 1999, Sherman Jackson (available at jstor)