Protestants know the personal presence of Jesus, because they recognize Him as their personal Lord and Savior (a liturgical formula, by the way). But there is only one place where He is always completely present, and every Christian in the world knew it until the Protestant Reformation: in the Eucharist.
Christians worship Christ because He is God. Catholics worship the Eucharist because the Eucharist is Christ. Under the appearances of bread and wine, He is literally just as fully and truly and really and objectively present to the Catholic who adores Him there and who receives Him there in Holy Communion, as He was to His disciples in Palestine for three years. In fact, more so, until the Last Supper, when they too were privileged to eat His body and drink His blood and get Him inside themselves instead of just outside.
I think this is the most controversial of all the Catholic doctrines. It generated the most controversy and passion and war at the time of the Reformation. Read the history of the wars of the Reformation in primary sources and you will see this.
If Protestants are right, Catholics are the most ridiculous heretics and idolaters imaginable, worshipping bread and wine. Compared with the Eucharist, all the disputes about Church authority, Mary, Saints, Purgatory or Bible interpretation, or baptism, or predestination are almost trivial.
On the other hand, if Catholics are right, then Protestants are mission out on the most perfect, intimate, and complete union with their Lord that is possible in life. Christ is knocking at their door, and they're not opening it because they deny it is really He. They are like 1st-century Jews who rejected their God when He appeared to them in the flesh because they refused to believe that God would come in disguise; they didn't have the faith to see the invisible God in the visible human appearances. Hence, it is they who are the idolaters, being hung up on the physical, the creaturely.
I believe most other Catholic-Protestant issues have been negotiated and agreed on. Thoughts?