Goodness.....
Right now I woukd be say a faithfulness to Scripture.
Many of our differences are related to traditions. I believe if we could trust in the words of God rather than understandings we would be more unified (granted, different interpretations would exist, but Jesus would make us stand - these would be interpretations of "what is written").
Culture is another aspect I would address. We tend to segergate by age, race and gender. But the young can learn from the old (and vice versa), race is a social construct (we are one race, a chosen race, a priesthood of believers).
Secular politics would probably be 3rd on my list.