1.Sunni muslims are pure monotheists while shias are not.Shias give some of the attributes of God to humans(12) Imams) and also invoke them as well which is Shirk.Shirk is the most biggest sin in Islam.
2.Classical scholars of Shias(Not all) believed Quran is distorted while Sunnis believes its not. They believed this becasue they were not able to find evidence of their man made religion in the Quran. Thats why they says its not pure. Some companions had removed verses of the Quran which supported their Shirk.
"Allah said in the Quran that He has send the Quran and He will prevent it from corruption". If we take the view of the shias,that makes Islam false.
3.In Shiaism,prostitution is legal.Its called Mutah(Temporary marriage). You can have an hour or 3 hour or days of temporary marriage.Its for pleasure only.while Sunnis dont practice it and consider it adultery.
4.Shias practice "Taqiyyah". Taqiyyah is a major part of their religion, almost 9/10.Taqiyyah means "Lying or concealing the truth" inoder to spread their religion and prevents lay shias from the truth. While Sunnis dont practice it and consider it a major sin.
5. The common source between Shias and Sunnis is Quran.Quran refutes almost all the man made thoeologies of the Shias.
6. Fatimah RA is worshipped in the shia Islam as well. She is like Goddess.