From the Catholic side has been 'we're almost there, what's keeping you?' From my talks with Orthodox priests, deacons and laity, the response seems to be 'when you abandon the idea of Papal Infallibility.' Really, that's about it. After all, that cuts against the crux of Orthodoxy's self identification. The Ecumenical Councils as opposed to Divine Revelation only through a single person. Which, by the way, is something some Orthodox point out that helped lay the groundwork for the Reformation. After all, if a single Pope can receive infallible revelation, why not a single German monk? There's a reason that Catholicism alone had a Reformation.
Not that there aren't other disagreements. Most of the bad blood seems to have gone away, as long as Catholics don't pull a 'serves them right' attitude that I've caught once or twice on the blogosphere. But on the whole, there isn't much holding them back but that one doctrine. Just how willing the Catholic Church will be to meet them half way and abandon Papal Infallibility remains to be seen. But as of now, it's a small, minor difference that, in the end, is a million miles wide, at least from the POV of most of the Orthodox I've gotten to know.