Thanks to the Protestant Reformation of the 16th Century, Catholics and Protestants have diverged on a lot of different beliefs. While the following points don't apply to all Protestants at all times, they certainly do to a large number of them. Protestants are great Christians, and they love Jesus Christ, for sure. There are MANY great protestants, and MANY bad Catholics. The great protestants who love scripture and try their best to live by it will probably be entering heaven before the bad Catholics who do not live up to their faith and who receive communion in the state of mortal sin. In other words, they do more with less. The items below are not meant to disparage anyone, but are only meant as points of departure for great Catholic - Protestant discussions of the one true faith. And if we don't discuss them, then how can we ever fulfill the prophecy of Jesus in John 17:21-22, that His followers may "all be one"?
Some Protestants see things as "either/or - You must love Jesus, but not Mary or the saints. "Faith alone" and "Scripture alone" are how they view salvation. Catholics certainly believe in loving Jesus, faith and scripture, but also include the necessities of the Church, Sacred Tradition, the Magesterium, the communion of saints, the sacraments, love, hope, charity, etc.
Some Protestants believe that their personal view of scripture is the only right way to interpret scripture (some might say infallible), even when it conflicts with other Protestants' view of scripture. And they will even claim that their personal view is the only correct view, at the same time they claim that no Pope can ever be infallible, because he's just a man. And this personal interpretation of scripture flies in the face of what Peter said about it in 2 Peter 1:20!
Some Protestants think that Catholics add beliefs to what is necessary for salvation; Catholics believe that Protestants subtract beliefs as to what is necessary for salvation.
Some Protestants see Mary as an ordinary person, just like everyone else. Catholics see Mary as being handpicked by God to be the New Testament Ark of the Covenant, the Mother of God who was always full of grace, the New Eve (who reversed Eve's action), a Woman Clothed with the sun who fights satan, and Our Mother. In other words, like Gabriel said BEFORE the incarnation,The Lord IS WITH Thee!
Some Protestants think that saints are dead and don't know what's going on down here on earth. Dead in the body = dead in the soul, they think. Catholics believe that saints are very much alive, know what we are presently enduring, present our prayers to God, and are equal to the angels.
Some Protestants do not believe that grace is administered through the seven sacraments. Catholics believe that the sacraments are one of God's ways of delivering His grace.
Some Protestants do not believe in the holiness of relics. Catholics believe that relics are sacred and have spiritual power.
Some Protestants believe in something called "the invisible church", which is a body of believers around the world who truly believe and follow Jesus Christ. The term "invisible church" is not found in the bible. Catholics are part of a very visible and active worldwide Church.
Some Protestants don't believe in apostolic succession, nor do they think it's necessary. Catholic priests, Bishops, Cardinals, and Popes have a direct lineage all the way back to St. Peter and the Apostles. Catholics view apostolic succession as one of the requirements for being the one true Church.
Some Protestants don't believe in the necessity of good works for salvation (faith alone). Catholics believe that good works are a necessary fruit of one's faith.
Some Protestants believe that the Bible alone is the pillar of truth. Catholics believe The Church is the Pillar of Truth, and the Church uses its Bible like a University uses its textbooks - to teach those hungry for the Word.
Some Protestants believe that the Eucharist is only a memorial or a symbol of Jesus. Catholics believe that the Eucharist is the physical Body and Blood of Christ, which also forgives sins.
Some Protestants believe that traditions are bad. Catholics believe that Christian traditions are good and necessary.
Some Protestants believe that the Genesis creation took place in six 24 hour days. Catholics don't really care, as long as one believes that God created the world, that Adam and Eve really existed, and that their sin led to the downfall of man in the Garden of Eden.
Some Protestants see only the end of the world in the book of Revelation. Catholics believe that a lot of the book of Revelation refers to the fall of Jerusalem in 70 AD.
Some Protestants believe the Pope is the Antichrist. Catholics believe that the antichrist is anyone who denies that Jesus is the Christ, which would exclude every Pope.
Some Protestants believe that the whore of Babylon is the Catholic Church. Catholics believe that it is a city that martyred the saints, like Jerusalem.
Some Protestants believe that no people are righteous. Catholics believe that Zechariah, Elizabeth, and Simeon in Luke's gospel are righteous, as are Catholics who are in the state of grace.
Some Protestants don't believe in the necessity of the confession of sins to a priest, because the blood of Jesus covers their sins ahead of time (sounds like an indulgence!). Catholics believe that the ordinary means of forgiveness of sins is the confessing of sins to a priest.
Protestant bibles only have 66 books, since Luther threw out 7 books of the Bible he didn't personally agree with. Catholic Bibles have 73 books in them. They have been in the Catholic Bible from the late fourth century until today.
Some Protestants believe that the Bible is self explanatory, a doctrine that is not found in the Bible. Catholic believe that the Church, the creator and originator of the Bible through the inspiration of the Holy Spirit, has the authority to properly interpret scripture.
Some Protestants believe that drinking wine is evil. Catholics believe that Jesus changed the water into wine at the wedding feast at Cana, and that this is a good thing. Getting drunk on wine, of course is evil.
Some Protestants don't really believe in biblical typology, where a person or an event in the Old Testament foreshadows a person or an event in the New Testament. Catholics see this as a primary tool of biblical exegesis.
Some Protestant Churches are pastor based churches, where the congregation loves and believes in the personal beliefs, charisma, and speaking style of their Pastor. Billboards advertising their church may have a huge picture of the pastor on the billboard. Catholic Churches are Eucharistic-based churches, and the priest is not the main attraction. The Eucharist is.
Some Protestants cannot see any Catholic beliefs in the Bible (veneration of Mary, the Eucharistic presence of Jesus, the communion of saints, typology, statues are Ok). The Catholic Church, the originator and creator of the Bible, sees all of these things.
Some Protestants view the Church as a meeting place of believers, for bible reading, singing, fellowship, and preaching, and they are free to pick and choose different churches based on what they personally feel to be true. Catholics view The Church as a holy meeting place where the Eucharist and the other six sacraments are dispensed by God's priests to give us His sanctifying grace. The first half of the Mass (prefigured in Malachi 1:11) is the Liturgy of The Word, the second half is the Liturgy of The Eucharist.
Some Protestants love to quote scripture that proves their point of view is correct. However, they do not like it when Catholics quote scripture back to them that proves the Catholic view is correct. They most likely will get offended at you for trying to evangelize them with scripture and will shut down the conversation when that happens.
Some Protestant Churches believe that you can change the definition of sin by a simple majority vote of the elders, like with abortion, artificial birth control, and homosexuality. They do this to be more "relevant" to the modern world. The Catholic Church does not wish to go along to get along with the world.
Some Protestant pastors feel no guilt whatsoever about making huge salaries. Catholic priests do not make huge salaries.
Some Protestants like to accuse the Catholic Church of being anti-science, because of one and only one thing, the Galileo affair (Galileo was funded by the Catholic Church, and he had not adequately proven his theory of heliocentricity to the Church. Galileo was wrong about the earth's orbit causing the ocean tides, and he was also wrong about the earth's orbit being a perfect circle). For the record, the Catholic Church has funded thousands of science projects throughout the centuries, including the huge Vatican Observatory. Gregor Mendel, who discovered genetics, was a Catholic priest. The only reason any science at all made it through the dark ages was because of the Catholic Church hiding all of the textbooks from the rampaging and pillaging Huns, Vandals, and Visigoths. They will accuse the church of being anti science at the same time they deny scientific theories like evolution. 
Some Protestants like to say that the Catholic Church is evil because of the Inquisition and the Crusades. They conveniently ignore all of the Catholics killed by Protestant leaders in Ireland and England, as in the Irish Martyrs and the English Martyrs. And they will never bring up the alleged witches killed by Protestant leaders in Salem Massachusetts. And then there was Jonestown and Waco.
Some Protestants believe that you have to have a personal relationship with Jesus Christ as your personal Lord and Savior. Catholics consume Jesus - Body, Blood, Soul, & Divinity - in the Eucharist. You can't get any more personal than that.
Some Protestants will give you leaflets to read that explain how they got "saved", and how they think you should also. When Catholics give Protestants something to read, especially if it talks about the Mother of God, they will never read it, and instead will usually recoil in horror as if the devil was after them. Catholics don't believe that salvation happens until the moment of death, because, as Paul says, it is a race that has to be run until the end.
Some Protestants believe that any and all statues in a church is idolatry. Catholics know that there were 2 statues of angels on top of the ark of the Covenant which carried the 10 commandments, that Solomon's temple had statues of cherubim in it, and that the Israelites in the desert were cured from snakebites by looking at a statue of a serpent. It is only a sin if one worships the statue as if it were God, like when the Israelites worshiped the golden calf.
Some Protestants do not understand the difference between veneration and worship. They think that to venerate a saint, like the Mother of God, is to "worship" her. Catholics know that Jesus himself venerated Mary as His Mother, because of the Commandment to "Honor thy Father and THY MOTHER." To venerate someone is to show that person honor and respect. To worship God is certainly MUCH more than that.
Some Protestants believe that if it isn't in the Bible, then it can't be true. Catholics believe that there are many things that aren't specifically mentioned in the Bible that are true. Protestants have a mental block about reading what the early Church Fathers had to say (it isn't scriptural), but have no problem reading Billy Graham, Max Lucado, and Martin Luther.
Some Protestants believe that man has no free will, that you are either predestined for heaven or for hell. Catholics believe that we all have free will. If not, then why evangelize anyone? What good would that do if everything is already predetermined?
Some Protestants think that every Christian is a priest (the priesthood of believers), and see nothing special about Catholic Priests. Catholics recognize that Catholic Priests are consecrated (set apart) so that they can give us all the Eucharist and Absolution in the sacrament of confession.
