I might not fall out too greatly over transubstantiation, as although I believe this to be fundamentally wrong, it has at least arisen from a literalist interpretation of the Scriptures which I can respect.
Papal infallibility is a non starter; the 'creation of saints' is a non-runner - indeed I am a saint by biblical definition!
Interceding for the dead is wrong as is the over attention to relics and calling the priest 'Father' which is anti-scriptural.
But above all of these things, it is the mariolatry which offends me most.
The fictions about Mary abound.
She did not remain a virgin as at least 6 other childbirths prove and had she done so, her unconsummated marriage with Joseph would not have been a marriage at all - even in the eyes of the Roman Church!
She was not 'assumed into heaven', the Scriptures do not tell us to pray to dead people - quite the contrary - she 'does not intercede with her Son for us' - we as believers have a direct line to the Godhead and most emphatically she does NOT perform miracles or answer prayers!
Any miracles done in her name which receive a response - well - that response will have come from somewhere quite different altogether!
I am also offended by the expression 'Mary Mother of God' which gives an erroneous emphasis; is bordering blasphemy and misses the point. The fact that Jesus is God the Son and that she was His mother is the correct emphasis.