The most important trait was loyalty to their fellow men for other men. If one could show loyalty, one was a friend for life. Relationships were based upon duty, not common interests. Men who were brave, loyal, and trustworthy were revered. The expectations for men were to defend the territory or kingdom, for example, or work to provide for their families.
When Grendel begins attacking the Hrothgar's hall, men are stationed each night in it to try to thwart the attack. They put their lives on the line to defend Hrothgar's kingdom. Many, many die as Grendel continues his attacks. The deaths of these men were seen as honorable because they died defending their kingdom. Beowulf was the ultimate warrior, then, because he was able to defeat Grendel. He had all of the characteristics of a "manly" hero, including great, almost inhuman, strength, a sense of honor, trust, and loyalty. In fact, he came from another kingdom simply to slay Grendel for the Danes!
Kings were expected to be very wise and intelligent and to make sound decisions, in addition to the other qualities. They had to be calm in times of crisis and be trustworthy to all of their people to make the kingdom feel safe for its inhabitants.