Replying, Editing, and Voting
Replying to a post
Under any post you can see, click Reply to respond directly to it. Your reply is nested underneath that post, so readers can follow the back-and-forth of the conversation.
You can only reply once to any given post, and you can't reply to your own post. As with top-level responses, this is meant to encourage one thoughtful reply rather than an argument played out across many small posts - if you have more to say, look for the specific point you want to rebut and reply to that instead.
Expanding a discussion
Replies are loaded on demand - click the expand toggle on a post to load its replies, and "Load more" to see additional ones. Use the Following only checkbox above the discussion to filter down to threads you're following (see below).
Editing a post
You can edit a post you authored only before anyone has replied to it. Once the first reply arrives, editing is locked so that replies always stay accurate to what was originally posted.
Following a thread
Click the follow control on a post to be notified (via the activity bell) about new replies to it. Unfollow at any time the same way.
Voting on posts
Every post can be voted on using a set of facets that describe the quality of the argument - for example, whether it's Thoughtful, Strong, Intelligent, or Off-topic. (The exact list of facets may evolve over time.) Open the vote control on a post and choose the facet that best describes it. You can't vote on your own posts.
Votes are shown as a small pie breakdown on each post, so you can see at a glance how the community has assessed it.
Karma
Karma reflects how the community has voted on your posts over roughly the last year, broken down by facet. Frequently earning a facet's votes can earn you a badge shown on your profile - see Your profile: public vs. private. Karma is meant to reward consistently thoughtful, strong contributions - not to be gamed, so vote on posts honestly (see Rules for civil debate).