I like that comment a couple of posts back which refered to some of the qualities needed for effective leading and sharing in groups - like knowledge, trust, prayer and love!
I am sure that any list of factors should reinforce such qualities at (3) and (4). Perhaps -
(3) Gifted group leaders. Among other qualifications we should include: good preparation with flexibility and willingness to listen to others; pastoral concern for every group member; discipline with time to keep balance between teaching/interaction and introverts/extroverts (!); love that doesn't miss the main point of helping fellow disciples grow. Of course, like many leadership gifts, these need to be learned and nurtured.
And at (4) Gifted group members. Yes, much depends on the leaders, but the willingness of group members to commit to each other is absolutely vital too. This too is a matter of God's gifting. Their preparation, ability to listen, pastoral sensitivity, self-discipline and love is essential for effective group life.
What else needs to be added to my priorities 1-4?