Dear Till,
I would not presume to speak for other Orthodox, least of all on this forum which is the home of what is, for conveniences' sake called Eastern Orthodox; for one thing, they are more than capable of speaking for themselves; for another, I know how easy it is for the holding of traditional Orthodox ecclesiology to come across in the way you have described it, almost as an equivalent of that Protestant or Catholic declaration that they are the one true Church.
My own bishop is fond of the formulation one often sees in Orthodox literature, which is that whilst one can be sure where the Church is not, only God is sure where it is. It may well be that sometimes, in the sheer joy of the encounter with the Risen Christ in the Orthodox Church that we do sometimes give the impression that He is only with us; if we do that, then we fail in humility - or at the very least, in clarity of expression.
We know the Holy Ghost is with us, and we know that despite the many travails through which the Orthodox Church has passed and is passing, the Gates of Hell will not prevail against it; but that ought not to lead us into prideful attitudes; we should boast in naught save the name of Christ, and Him Crucified and Risen. To those to whom much has been given, much is expected, and there is none of us, no, not even one, who is righteous. So, if an impression of pride has been created, it is meet and right that we should say that any pride is only in Christ.
That said, the Orthodox do, of course, hold what others do in their turn, which is that the fullness of the Faith is to be found within the living Holy Tradition of the Church; but I hope we do that not in a spirit of denigrating others, but in one which simply expresses the hope that the healing which we have found, might also be found by others. And, of course, we can only be sure that that healing is found within the spiritual hospital which is the Orthodox Church. But we are not, I hope, unmindful that others may hold the same of the place where they encounter the Risen Lord.
My own personal hope is that others find what I have found in Orthodoxy; if they find that where they are, who am I to think I can set a limit to the work of the Spirit? We can know only what we experience; but we ought not to forget the same is true of others.
What does make us wary, and that was what Protoevangel was, I suspect, getting at, is the notion of syncretism. However unfashionable it may be in parts of the Western world, there is a revealed Truth, and it is a person, the Risen Christ. Any attempt to water down to a lowest common denominator the 'faith once given', does threaten the means of salvation; this is why 'false ecumenism' meets such a fierce response from the Orthodox.
That true ecumenism, which consists in finding what others who confess the name of Christ hold, does not seek unity as its goal, although it may, if it is God's will, find it as a consequence of straight talking - and listening to the same.
Peace,
Anglian