Catholics believe that the church (e.g. its priests, its sacraments) is essential to salvation. Protestants believe that the essential relationship is between each Man and God - the church being variously more or less helpful; but not absolutely necessary to salvation.
I think this is the nub of the disagreement. When stated thus baldly I find I do not believe the church is essential to salvation. So I suppose I am 'a Protestant', by the above definition.
Why? Because I honestly cannot (I tried) believe that our loving Father - the creator - would have set-up this world on that basis.
I am, however, in agreement with the Catholic, especially Orthodox, focus on theosis (or progression towards a greater state of divinity) as the main business of life; what we are supposed to do. We are, I think, meant to make the choice that is salvation, which is something personal between each Man and God - and then to embark on theosis as the main focus of living... which for many people in many circumstances, leads to a church.
However, any specific church (or church-situation) may be either helpful or a hindrance (indeed a threat) to theosis - and may even attack the conditions of salvation.
So this, again, is a very Protestant attitude of mine; that ultimately I judge the church (in both general and specific manifestations) by the deepest discernment I can attain; not vice versa.
Therefore, I have to say that The Reformation was A Good Thing - despite everything! A good thing because its main point was true.
NOTE: I am talking here about the difference between Catholic and Protestant denominations as ideal types. In practice, many individual Catholics are Protestant, by the above definition - and practising Protestants (perhaps especially in Lutheran or Anglican churches) and members of new Christian groups (such as Mormons) may be 'Catholic' in their personal beliefs or assumptions.