From Social Patterns
Comments that are related to previously posted comments and may only be tangentially (or not at all) related to the main content they are connected to.
Use this pattern when you expect or wish to encourage offshoot conversations that may arise from the main content. This is obviously very closely related to the comments pattern but threaded comments tend to quickly move away from the original topic into areas that the original author did not originally anticipate.
- Add the ability to reply to a specific comment in addition to allowing comments related to the main content body
- Offer users some way of quoting the comment they are replying to
- Indent replies under their parent comment, thereby keeping related comments together
- Provide uses with a way to 'collapse' related groups of comments they are not interested in seeing
The comments pattern is an excellent way of adding social interaction, however it tends to focus comments sharply only on the main content in question. Threaded comments add significant social interaction between users as it allows them to conduct multiple conversations through comments that may or may not be related.