I agree - I'm not sure whether to call what I find most challenging "Interaction Design" or "User Experience," but there are a lot of elements of design that are neither technical nor aesthetic. In the chatroom example, there isn't much involved because it appears to be so generic, but as you innovate more and more, this ID/UX becomes increasingly challenging.
One slightly related design problem would be, for example, how the buddy list on facebook chat works. One could classify users into groups, allow you to 'pin' a user at the top, expose more ways to have group conversations. While, one could always argue for the simplest possible solution (i.e. one big list!), it's not always the best for the user and I would read anything that helps in guiding processes to discover and decide what is best for the user. In my experience, good UI is often far less controversial than good ID/UX and less of an issue when keeping a team happy and excited about a product. All of the biggest issues where I work have been on a much bigger paradigmatic level that we debate on philosophical grounds endlessly to little avail.