As I see it (and this is only my opinion), the primary issue was Christology (whether EO and OO Christologies were equally Orthodox). The next issue is closely tied to all this, IMO, and that is the recognition of councils, and how that is to be resolved. Both EO and OO liturgical texts anathematise the other (I've seen examples from both sides), and this extends to certain saints, and that is also an issue, but I think that could be the easiest to resolve once the first is definitively taken care of. I am not sure there are any others. The second and third are definitely not resolved yet; the first is there, or almost there, and I praise God for that, since it is the biggest hurdle.
Is "cultivating a welcoming place for liturgical diversity within the EO world", as Brendan put it, really an issue for the EO to solve before any reunion? From the EO I've spoken with, clergy and laity, it doesn't seem like the EO would have a problem with differing liturgical rites; but does the almost exclusive use of the Byzantine Rite throughout EOxy render the EO unable to allow other rites? I would think the existence of a "Western Rite" in Eastern Orthodoxy argues against this, but perhaps it isn't well received? "Byzantinisation" has happened in the past (Alexandria and Antioch, for example), but is it a real possibility in the present day? Would the wider EO communion really take issue if, for example, the Greek Patriarchate of Antioch adopted the Syrian Rite that the Syrian Orthodox use, even if they tweak it a bit (deleting certain saints, adding others, etc.)? Would they insist on Oriental Orthodox adopting the Byzantine Rite? Is this really a significant issue? I would not have thought so, but I am not EO, so I wouldn't know.