when you write a longer comment to someone's question or answer you don't get any formatting options. A simple line break would make the comments a lot more readable.
Comments are for quick notes about a post. If you need something more than that, there is the chat; you can create a room (if you have enough reputation) and leave a comment to the post, inviting the user to that room.
As HedgeMage said, comments are not for discussions; if two users start a discussion in comments, an automatic message offers to move the discussion on chat, and if one of the users click on the link, a chat room containing the copy of the comments is created.