Like the title says, I think a list of participants should be shown on the conversation list page. Otherwise, there is no way to tell who all is involved in a conversation until you click on it. That is very time consuming, especially for members who use the conversation system regularly and cannot remember who is involved in each conversation simply by looking at the title.
Perhaps even a button to click on that will use ajax to load a "popup" list of conversation participants, or something.
Sorry if this has already been suggested. I did search, and found nothing (although that's not abnormal for me).