A bunch of considerations all working together.
The key difference, it seems to me, is that Catholicism defines the Church first and foremost as a fellowship of people, with a personal center of unity (the Papacy). Yes, purity of doctrine is important. But in Orthodoxy purity of doctrine and liturgy seems to be the primary criterion, to the point that the people who make up the Church disappear from view. So, for instance, Catholics have in the past century been forced into an ecumenical ecclesiology–admittedly with much kicking and screaming–because they have had to recognize that there are genuine Christian believers outside the visible bounds of the Roman Communion. The Orthodox, faced with the same obvious reality, simply say “well, there probably are real Christian believers outside the bounds of our Communion, but that doesn’t affect our ecclesiology–after all, we know where the Church is but not where it isn’t, etc.” Thus, Catholics can–indeed must, if they take Vatican II and other recent developments seriously–acknowledge that there are elements of Christian faith and practice outside their borders which not only have validity as imperfect imitations of Catholicism, but which may in fact represent things Catholics need to learn from. The Orthodox view of Catholicity, on the other hand, is a perfect circle of more or less static doctrine and liturgy, with an imperfect recognition of the reality of development and the valid insights of other Christians. So I think Orthodoxy functions very well as a standard for the rest of us to look to to make sure we aren’t getting too far away from the core of the Faith. But the Roman Communion does a much better job of defining where the boundaries of the Faith are. (Obviously, since I’m still Episcopalian, I don’t entirely accept even the boundaries drawn by Rome–but I respect them.)
The other factors that RCs typically point to, like the greater cultural universality of their Communion, the fact that they are in communion with fairly large Eastern churches and have, at least in recent years, made serious efforts to “breathe with both lungs,” or their relatively greater ability to resist cultural assimilation–these should all be taken seriously as well, though they may not be the knock-down proofs that many RC apologists claim.
Edwin