Secondly the differences in Muslims are similar to that of other organized religions which include Christianity and Judaism.
There is discord in every faith, indeed it was predicted that there would be as many as 72 sects in Islam (sadly).
However, when it comes to the fundamentals I must say that Islam is more united than most other faiths, especially the Abrahamic faiths.
Every Muslim acknowledges praying 5 times a day, accepting prophethood of all the prophets, fasting, giving to charity, going to Hajj, believing in the revealed books etc etc. whether or not they practice it is a different matter, however they certainly view them as duties.
Our concept of Allah is also the same, shiahs and sunnis and all the sub-sects thereof have as their primary differences little issues that do not necessarily conflict with their respective core principles.
However, in Christianity for example they can't even decide whether or not Jesus is god. I have conversed with many Christians and no two come up with the same formulations with regards to God and salvation. They have thousands of versions of the Bible, whereas today all the Qurans you buy are exactly the same. One who has memorised the entire Quran (hafiz) from the USA could sit in front of a hafiz from China and their recitation would be exactly the same.
A Pakistani Imam could go to any mosque around the world and lead the Friday prayer without any confusion in the alien congregation.
In Christendom there is great difference with regards to prayers and they do not even pray like Jesus and all the prophets before him prayed. Some dance about, some sing, some go crazy, some start talking gibberish (talking in tongues) and some just sit and watch –
when in actual fact Jesus (upon him be peace) and all the other prophets, including the last of them all (peace be upon him) worshiped Allah in exactly the same way. I can prove this from the Bible upon request.