1- The first of which is that it is the system by which all creations relate to their creator. The word Islam is originally Arabic which means "to submit to the will of God". Everything in the universe submits to God without choice. What makes mankind different is that although their body functions in submission without choice (heart beat, blood flow, digestive tract etc..) we have been given choice as to our actions based on intelligence. This trust of choice was offered to all creations. Adam, the father of mankind, was the only to accept it. This is why God commanded the angels to bow down in respect to him and his progeny who would choose to submit to the will of thier Creator through the message He would reveal to His Prophet's. The reality is that the majority of man was ignorant and unjust in carrying this responsibility. Two verses of the Qur'an which clarify this point are-
a) "Indeed We (God- in the royal We noting His Exaltedness) offered "the trust" of reason and personal choice to everything in the Heaven and Earth and they all refused and were fearful of such a responsibility, but it was Adam who took it upon his progney and they were ignorant and unjust toward that trust." (33:72)
b)"Indeed the only way pleasing to God is Islam (Submitting to His will) and the previous nations who recieved scripture and the knowledge of pure monotheism only differed after that in transgression between themselves..."(3:19)
2- Islam is a complete way of life which can truly change the lives and minds of people to pure morality as long as they adhere to it and increase their knowledge of the meaning of the Qur'an and the example of the Prophet Muhammad (pbuh) and his companions. If you truly studied the state of the Arabs before Islam you will see that they were polytheists, they were lawless and the murder rate was outrageous, their society thrived on alcohol and gambling, they were mostly illiterate and ignorant, they had hundreds of rediculous superstitions, they oppressed women and prostitution was worse than vegas and new york, they were a weak community and seen as under the control of the Persians and Byzantines, and many more.
Through the 23 years of the Prophethood of Muhammad and wthin 30 years after his death the Arabs had become a unified nation of strict monotheists, they had a comprehensive code of national law based on the revelations, and murder, fornication, and prostitution became very scarce if not non-existant because of the death penalty for such crimes, alcohol and gambling were abolished completely, they not only became about 80% literate but began thriving in science, math, and medicine, women were treated as spiritual beings and were respected as our mothers and wives, women were given the right to vote, teach, own property, and have roles of leadership (13 centuries before the west began to think about offering such rights), and finally they toppled the tyrannic oppressive empires of persia and byzantium in this time securing the dominant spot in control of a third of the civil world for the next 10 centuries. Of course in the last few centuries the keen observer of history notes that the Muslim world has become stagnant and ritually symbolistic. We see ignorance, poverty, oppression and a lack of any oganization or leadership. This is because of their haven drifted from teh teaches of Islam as a daily practice and philosophy of life and reintroduced many cultural practices which were originally abolished by Islam. For this reason there are many revival movements working hard to pick up where their predecessors left off.
3- Islam is so great because it is the only way to eternal peace and happinness in the presence of Almighty God the Exalted. Many of our Christian brothers say, but Jesus said, 'I am the way, the truth and the life and no one will go to God except through me'. To thier surprise, I tell them of course he said those things and as Muslims we agree. All Prophets were sent by God to reveal His guidance to mankind that we may recieve the correct way to worship God in truth and earn eternal life in Heaven with Him and anyone who does not accept His guidance through them will be doomed for their arrogance and treachery to God.
Of course Jesus was only sent to the lost Jews as is even quoted in the bible today (Matt. 15:24) and his Prophethood was very short and we actually have very little information as to his teachings today as a result of this. His nation rejected him and the millions who claim to follow him today are really only Christian because of Constantine the roman caeser who became a Christian and used His power to spread his religion throughout the roman empire.
Muhammad was the final messenger of God and he was sent to the whole of mankind as the carrier of the light of God's Guidance of Mercy. Some verses of the Qur'an in relation to this point are-
a) "And we have sent you to all of mankind as a bringer of glad tidings (of God's forgiveness to the believers)and as warner (as to the punishment for those who reject). And most people don't know."(34:28)
b) "God sent the messenger (Muhammad) reciting to you the clear distinct verses of God. This is that God may bring those who believe from darkness into the light. And whoever believes in God and does righteousness, God will admit them into an eternal Paradise of gardens with rivers flowing therein. And God has perfected for them their provision."(65:11)
4- The final great thing about Islam is in it's book which proves itself to be divine and doesn't just rely upon faith. In addition to that it has been preserved to the letter since it was revealed which is something no other revelation achieved. The reason for this is because it is the final book sent to all of mankind so it must be preserved that billions of people will not come on Judgment Day saying we didn't know.