Commitment
It is imperative that the members of Man2Man are committed. Devoted to the group, and to each other. Without a solid commitment of people saying, "Yes, I will attend all meetings, ready and willing to challenge and be challenged, the group will flounder.
Fire
Man2Man is a support group where men gather together and discuss their feelings. Feelings should be discussed, absolutely. But Man2Man is more than that. Man2Man has fire. If the men in Man2Man are willing to challenge each other, hold each other accountable, and encourage action and momentum in each other's lives (as opposed to endless, circular emotional processing that doesn't go anywhere), then the group will struggle.
Confidentiality
Private matters (regarding relationships, career, sex lives, and emotional realities) get discussed in here in Man2Man. Therefore, confidentiality is a necessary element. Members are allowed to talk about their own process outside of group (to friends, their partners/spouses, etc.), of course, but they should avoid talking about anyone else's process, and especially mentioning the names of other members when referring to any happenings within the group.
Courage
It takes courage to really show up and be seen in Man2Man. Courage to let your inner world be known to other men. Courage to challenge men both older and younger than you. Similar to the element of fire (and overlapping with the concept of vulnerability), courage requires that men both be willing to let themselves be seen fully, and be willing to see/honour/challenge other men in the circle fully, when appropriate.