In fact the lists are moderated for users that post from not-subscribed email addresses. However, this moderation should be transparent to the poster, you only might notice some additional delay until your message shows up on the list, as it has to be handled manually. Note that this is a one-time delay, as the address will be added to a white-list, so that further messages from the same sending address will get through immediatelly. The same holds for people who are sending their first message after subscribing to a list. These two measures greatly help to keep the lists' spam level low.
So, to initiate a discussion you do not need to subscribe to the mailing list in question. And people are very much encouraged to maintain the CC recipients, so that you'll also receive replys to your message.