It is Islam with a "but…"
In his notes on The River War, Sir Winston Churchill wrote:
“How dreadful are the curses which Mohammedanism lays on its votaries! Besides the fanatical frenzy, which is as dangerous in a man as hydrophobia in a dog, there is this fearful fatalistic apathy. The effects are apparent in many countries. Improvident habits, slovenly systems of agriculture, sluggish methods of commerce, and insecurity of property exist wherever the followers of the Prophet rule or live. A degraded sensualism deprives this life of its grace and refinement; the next of its dignity and sanctity. The fact that in Mohammedan law every woman must belong to some man as his absolute property – either as a child, a wife, or a concubine – must delay the final extinction of slavery until the faith of Islam has ceased to be a great power among men. Individual Moslems may show splendid qualities. Thousands become the brave and loyal soldiers of the Queen: all know how to die: but the influence of the religion paralyses the social development of those who follow it. No stronger retrograde force exists in the world. Far from being moribund, Mohammedanism is a militant and proselytizing faith. It has already spread throughout Central Africa, raising fearless warriors at every step; and were it not that Christianity is sheltered in the strong arms of science, the science against which it had vainly struggled, the civilisation of modern Europe might fall, as fell the civilisation of ancient Rome” - The River War
So what is Mohammedanism? Is it a new religion or a sect? As far as I can tell...
Mohammedanism is a religion which claims to be based on the Qur'an, but is actually based on the supposed Sunna and Hadith of Prophet Mohammad s.a.w., which distils the expansive Deen of the Qur'an to 5 personalized rituals, even though these are not identified as such in the Qur'an.
Because it claims to be based on the Qur'an, most adherents of Mohammedanism call themselves Muslim. Non-Mohammedans are all kafir. An unintended result of this is that non-Mohammedans frequently confuse Mohammedanism and Islam, and think that Mohammedanism is Islam. To be fair, though, even Muslims are sometimes confused.
One of the tenets of Mohammedanism is the exalted status of the Prophet s.a.w. above all other prophets, and to deny this is to deny Revelation, and the denier is considered kafir. Clearly this is a deviation from the Qur’anic edict that Muslims do not differentiate between God's prophets (2:136, 2:285, 3:84, 4:152, 17:55). All prophets are equal, but…
Mohammedans consider the Hadith close to divine (and some, the Qudsi hadith, divine) scripture, and anyone who rejects (or even questions) the hadith automatically rejects Muhammad s.a.w as a prophet, and is considered kafir.
The Qur'an asserts it is sufficient as Guidance for Islam (5:3, 6:114, 7:52, 11:1) and nothing has been left out of it (6:38), Mohammedans say "it is complete, but…" because they have imposed their own meanings on the words the Qur'an uses and do not find detailed explanation for these new meanings in the Qur'an.
While the Qur'an is expansive, it is also practical and grants a lot of leverage in the practising of its commandments. The personalized rituals in Mohammedanism, however, are very rigid and constrictive, and unless one follows these rituals to the letter, the ritual will not be "accepted", or God will be angry. This is probably because these rituals have been quantified with personal rewards for each action or following of the Sunna. The Qur'an is complete, but…
While the Qur'an denounces sectarianism (6:159), the Hadith teach Mohammedans about the single saved sect out of 73. While the Qur'an disavows superstitions and magic (4:51), the hadith teach that women and donkeys are bad omens. And while the Qur'an says call on your God in secret (7:55), Mohammedans advertise their prayer times loudly with Azan. Sects are not allowed, but…
While the Qur’an simply asserts that you cannot marry until you’re sound of judgment (4:6) and able to enter into a contract (4:21), Mohammedans spend half their time denying the Sahih Hadith that claims Aisha married the Prophet at 6 or 9, and the other half of the time claiming Sahih Hadith are scripture and establish Laws enabling child marriage.
Ironically, the hadith which mention the five pillars sometimes contradict each other in the shahadah (without the prophet, and with), but the Mohammedans have chosen the shahadah which mentions the name of their idol alongside Allah. A separate sect of Mohammedans have even added yet another phrase to the original shahadah in the Qur'an (There is no God but Allah) but of course, the Mohammedans consider this sect kufr. See constrictive rituals above.
Their aim in life is to "convert" everyone to Mohammedanism by any means and proselytize incessantly, sometimes even by force, or more often by trickery. They show the subject the Qur'an in all its beauty, inclusiveness, practicality, and egalitarianism, then once the subject says the shahadah, hit him with the stuff from the hadith because the Qur'an is considered inscrutable and complicated, and inaccessible to ordinary mortals.
As a result, even the smallest decisions have to be referred to hadith, except that there are too many of those. So, they have to ask an "Ulama" who is well versed in the hadith. For 1,200 years, Ulamas have made a living out of issuing fatwas (non-binding legal opinions) for just about anything under the sun, which anyone with an ounce of intellect, guided by the Qur'an, would have been able to decide for himself. They forget that if you are guided by the Qur'an, God is Al-Rahman Al-Rahim.
At this point, the subject often wonders if he has made a mistake, and remembers the oft-quoted phrase from the Qur'an 2:256 "There is no compulsion in religion". The Mohammedans tell him that that only applies to non-Mohammedans, and no one is allowed to leave Mohammedanism, as apostasy is punishable by death. Trickery. There is no compulsion, but…
Mohammedans often cannot differentiate between culture and religion. As a result, Arab culture is held aloft as the prime example of devoutness - their language, their dressing (which strangely applies to women only), their food (ajwa dates) and drink (camel urine... ok, kidding, Zamzam water). Islam transcends all cultures, but…
They encourage everyone to recite the Qur'an in Arabic regardless of whether the reader understands or not. This is OK because the religion comes from the hadith, and the Ulamas are the ones actually providing guidance.
Feeding the Arab beast is recommended, so Mohammedans will spend a fortune on going to the Hajj even if they cannot really afford it. If they have just enough, they will spend this on the ritual, as opposed to spending it on feeding the poor, because the personal ritual, as all personal rituals in Mohammedanism are, is paramount.
Only God knows how many Mohammedans there are at this point in time, but one thing for sure is that Prophet Muhammad will exclaim to God on judgment day, "My people have abandoned the Qur'an" (25:30).
I’ll end this by using the phrase which is the hallmark of Mohammedans - after every article explaining the details of the religion derived from the Hadith they write this...
Allah knows best.
(In Mohammedanism… our ulamas know better)
Addendum: Here's an extensive list of differences