1) willingness or desire to contribute (hopefully everyone does
2) the discipline of the participants (I find a growing trend of people wanting to talk over each other, start talking before someone has finished their thought, people wanting to dominate the conversation)
I’ve had conversations that are intolerable with 2/4 people because they’re either hard to talk to or talking over you, conversely I’ve had lovely conversations with 8 people that have gone extremely well.