Er, well... then there's that whole infallibility thing, and that whole pope thing... Not so easy.
Well, those too (LOL).
Seriously, I think, — just my opinion, — that the relations between the Protestants as a whole and Catholics could be healthier. This does not mean a sacramental union, like one we may accomplish with the Eastern Orthodox hopefully in a generation or two.
The Catholic-Protestant relations could be healthier if either side should
1. Refrain from describing the other confession in caricature terms. For example, the Protestants should listen to how the Catholics themselves explain the veneration of Mary, the saints, and the relics, rather than assuming that all these are idolatry or even detract from the worship of God. This doesn’t mean the Protestants have to venerate Mary, but it means that the Protestants express their disagreement in terms acceptable to the Catholics.
2. Concentrate on its own confession rather than on defects in other confessions, just like Protestants do between themselves, and the Catholics do with the Orthodox.
3. Acknowledge that each side is profoundly concerned in correctly understanding the entirety of the Holy Scripture, and that we have honest differences of opinion regarding the interpretation of some passages. For example, the role of good works in salvation in relation to professed faith, or the role of Mary in the salvation history are such differences. It is unhealthy to call another side’s interpretation as unscriptural, deride it as “tradition of men”, etc. when your own side also has but an interpretation by other group of men. To put differently, to offer a variety of interpretatins of scripture is one thing, and it is healthy. To say that one interpretation is inspired by the Holy Ghost and the other is not is not healthy.
4. Acknowledge that the works of the early fathers of the Church is an important historical witness to the practices of the historical early Church. It is fine to disagree with them here and there, unhealthy to create myths of the historicity of one side’s interpretation of scripture in absence of patristic evidence of such.
There is probably more, but these would be very good ground rules of ecumenical conduct.