What makes the Ultramarines special
Roboute Guilliman did not want to be a famous warlord. To him, war was simply a means to an end. He wanted to raise up a great Empire where all citizens were treated equally. Where there existed a society based on the core principles of meritocracy and continuous progress to an ever brighter future. Bigger cities, a growing populace, new advances in technology, ever-higher standards of living for the citizens, exploration and colonization of the galaxy. This is what he wanted: civilization.
Make no mistake, I’m not saying that this approach is entirely moral either. Unlike the Salamanders, the Ultramarines would graciously accept horrendous civilian casualties if it meant achieving a higher strategic objective. For example, Uriel Ventris, Captain of the 4th Company of the Ultramarines was willing to subject an entire planet to biological and nuclear weaponry just to protect the borders of Ultramar from the merest hint of Tau activity.
In fact, the Ultramarines would have gladly put every single man, woman and child in an entire sector to the sword if they deemed it was for the greater good. They would commit acts of savagery that would be on par with the Night Lords if Roboute Guilliman convinced them that this would help advance their cause.
The don’t fight for the common citizenry of the Imperium (like the Salamanders). They don’t fight to repent and bring redemption to the name of the primarch (like the Dark Angels). They don’t fight for honor (like the Space Wolves).
No, they fight to achieve the dream of their Primarch. The ancient human dream of civilization.