That is our biggest difference in theology, sure. But there are many other differences. I think I agree with the EO that it is a difference in mindset more so than any one particular thing, such that we could say "if Rome would change its view on X, we would/could be the same church". Having been a Roman Catholic before I became Orthodox, the change is huge, but cannot be narrowed down to one thing or even a list of top 3 or 10 or however many things. Our ecclesiology is different, our theology is different, our soteriology is different, etc., and each of these things contains within it many other things (e.g., ecclesiology reflects theology, i.e., when the RC say that their Pope is the "Vicar of Christ" or the "Supreme head of the Church" or whatever). It's all connected, so I cannot pick only one thing.