As people we have choices. We can choose to be mean and discontent. On the other hand, we can choose to be peaceful and choose to coexist with each other for the benefit of everyone. Always finding differences reinforces the things that separate us rather than the commonalities that bring us together.
Although our world is large and diverse, it is important to remember that people generally share similar goals and ambitions. Those ambitions include material security, that is, having the basic goods that are needed to survive, like food and housing. Additionally, people seek inclusion with community and family. The differences, such as language and culture, are what give our world diversity.
Yes, what people have in common is more important than their differences. Sometimes it's much easier to notice and focus on what is different between you and someone else. However, although it can be more difficult, trying to see what you might have in common with someone can often overcome any differences that may exist. Everyone is different but by the same token you can usually find some kind of common ground and that is the building block for a positive relationship.
People have a massive amount in common with each other, even in different parts of the world, and people are very quick to set themselves apart in factions. Especially in politics, people often really are interested in the same thing, and only disagree on the way in which they should get it.
I think that what people have in common with each other is more important than what people have that are different from each other. As human beings, we need to all see the best in each other and try to work together to make a better society. Trying to understand each other is a first step.