The main one was that they all followed the same religion, which then unified them.
Another one was they were really accepting, they didn't do racism, or judge people by status,. Every one was free.
Finally, the laws and knowledge the Muslims had. The knowledge was at a great extent, especially in Spain and Africa. The first street lamps, airplanes, camera, and safe were made in the Muslim World. Algebra as well!
I'm saying this through research and visits to museums from elementary school.