Islam is very distinct.

"There is no god except Allah and Muhammad is a messenger of Allah." is the statement that qualifies someone as a Muslim.
(http://www.islamic.org.uk/shahada.htm)

The Jews said, "The Lord our God, the Lord is one."

Even the Sabeans said, "There is no God, but God."

The thing that makes Muslims Muslim is the faith in this part, "...and Muhammad is a messenger of Allah."

Muhammad did make a point in the quran and in his life that this was necessary to be a "believer," according to him.

Quran 63:1-4 talks about those that say, "We bear witness that thou art indeed Allah's messenger" falsely, and calls them "the enemy." No mention is made here of people that do not believe that there is no Allah but Allah.

Quran 48:29 says, "Muhammad is the messenger of Allah. And those with him are hard against the disbelievers and merciful among themselves." This shows that "disbelievers" are against Muhammad and those that are with him.

Sahih Muslim 33 says, "The Messenger of Allah said: I have been commanded to fight against people till they testify that there is no god but Allah, that Muhammad is the messenger of Allah, and they establish prayer, and pay Zakat and if they do it, their blood and property are guaranteed protection on my behalf except when justified by law, and their affairs rest with Allah." Here Muhammad claims that it is his duty to fight against those that do not believe that he is the messenger of Allah.

Sahih Muslim 5917 says, "Allah's Messenger called Ali [and said]: “Proceed on and do not look about until Allah grants you victory,” and Ali went a bit and then halted and did not look about and then said in a loud voice: “Allah's Messenger, on what issue should I fight with the people?” Thereupon he (the Prophet) said: ”Fight with them until they bear testimony to the fact that there is no god but Allah and Muhammad is his Messenger…”  Once again, fighting was to be based on the goal of forcing people to believe that Muhammad was the messenger of Allah.

Islam is very distinct. That distinction is that one must believe that Muhammad is the messenger of Allah. This is a doctrine that was enforced with the penalty of death.