I know one big issue involved baptism. I have no idea what Catholics do or whether it’s just certain “flavors” (if there is such a thing) that do it, but baptizing infants/children (and by sprinkling rather than immersion) was considered one example of their “doing it wrong.”
That, because in the (typical?) Protestant view, it doesn’t count unless you’re of age/maturity to understand what you’re doing and old John showed by example how it should be done.
That’s one thing that popped into my mind… might not apply today, I don’t know.
Not that I agree with the argument that infant baptisms aren't efficasious (sp?), but we renew our baptismal vows at every Catholic Baptism, and every Easter.