Reformed Reader has ten great reasons not to join the Roman communion. My own top ten list would probably be something like this (going off the cuff, and not in any particular order):
1) Worship of Bread
The host is bread - it's not God and shouldn't be worshiped as God. In the Lord's supper, the bread is the body of Christ, but in a non-literal sense (as should be obvious).
God is my Holy Father, not any mere man.
Jesus Christ is the only mediator between God and man.
Mary was blessed and saved from her sins. She is our fellow creature and not to be offered hyper-dulia.
5) Worship by Images
God has explicitly forbidden the making or religious reverencing of images.
6) Prayers to the Dead
Those who are dead in Christ are - you know - dead in Christ. Their souls are with God. They cannot hear you any more than you can hear them.
7) Blood of the Martyrs
How could I join a denomination that persecuted the church of Christ and has not repented of this?
The worst example of schism is Rome, who pretends to be the only true church. Joining Roman communion is participation in that schism.
Rome does acknowledge Scripture's authority with her lips, but then takes that authority back by making human tradition the lens through which Scripture must be read.
10) Judgment or Gospel
I have no desire to receive the judgments God will mete out against those who preach or follow another gospel. Rather, I must and will hold fast to the gospel that the apostles preached, which is recorded in Scripture.